- Story told exclusively interactively in a cleverly designed world
- It does not try to imitate a movie, trying to play the strengths of the medium
- believable, well written characters that are not fanservice
- Original gameplay ideas and lots of polish, trying to engage the player in many different activities instead of waves and waves of enemies
- excellent art design, especially early in the game 
 - excellent and satisfying sound design
 
 
In short: Despite the lack of fancy cutscenes, hundreds of plot twists and a hugely thought out mythology; despite the lack of a protagonist even, people feel much more part of the creation of this world, City 17, are much more immersed and invested in it than in most other games. 
 
This is Half-Life 2's greatest success, and one that has not really been recreated in the genre since, unless you count Portal.

I'm sort of in the same boat. I basically just missed the boat and decided to play through the game a few years after it was released. Games were making incredible advances in very short periods of time back then (and arguably now, but I digress), and I came away feeling like Half Life 2 was kind of prosaic. Solid, and the storytelling was sufficient, but it didn't impress me.

i hate the entire Half-Life series.
 
*raises flame shield*

WALL OF TEXT!

 
Anyway, put it this way, if half life 2 hadn't been developed, you wouldn't have the Left 4 Dead or Left 4 Dead 2. Hey, what about Team Fortress 2 or Counter-Strike Source??? I think its an amazing game in that respect and we owe it that one :P

" - Story told exclusively interactively in a cleverly designed world - It does not try to imitate a movie, trying to play the strengths of the medium - believable, well written characters that are not fanservice - Original gameplay ideas and lots of polish, trying to engage the player in many different activities instead of waves and waves of enemies - excellent art design, especially early in the game   - excellent and satisfying sound design    In short: Despite the lack of fancy cutscenes, hundreds of plot twists and a hugely thought out mythology; despite the lack of a protagonist even, people feel much more part of the creation of this world, City 17, are much more immersed and invested in it than in most other games.    This is Half-Life 2's greatest success, and one that has not really been recreated in the genre since, unless you count Portal. "

"

WALL OF TEXT!"

source engine /thread

I think it's overrated...

I think what it comes down to is the Gravity Gun.  When the game came out, the weapon was a new and unique way of interacting with the environment in an FPS.  Players were encouraged to be creative in their killing as opposed to just firing bullets.  Since then, we've seen more variations on the G-Gun, as well as Portal Guns, Time-Manipulating Guns, and other non-standard arsenals from developers trying to recapture that G-Gun excitement.  Like Final Fantasy VII it is ingrained in a lot of people's minds as superior to what it actually is because of when many gamers played it in their gaming life.  That's probably the reason why I hold Half-Life 1 in higher regard than HL-2.  Additionally, I'm afraid to go back and play System Shock 2 whenever it eventually finds a re-release.  Some games are best left to our memories.   
 
I personally can't bitch too much over the holes in the story because I view all video game plots with the addendum of "for a video game".  It's better than some and no worse than almost all others.  If the game is fun and keeps me wanting more is more important than whether or not it makes sense.  Modern Warfare 2's single player campaign was ridiculously stupid but I got a big kick out of it because the game was fun.  I can't remember anything about Killzone 2 and most of Halo is just a blur, but I still blasted my way through them and am looking forward to the newer installments later this/next year.  Also, I don't think it's really fair to compare Alyx, a game character from an FPS to KOSMOS, an RPG character.  A better comparison would have been to Elena or Chloe from Uncharted 2.  Also, not an FPS but still primarily a shooter.  

" Half Life 2 makes me feel nauseous for some reason, moreso than any other FPS I've played. I got to Ravenholm before getting bored and quit. I should probably godmode it to the end someday. "

Half Life 2 was one of the greatest games for me I've played in the last 10 years. I can't name one specific reason for it, it's just how I felt after I finished it and still do as to this day. I'll try to get some points across to explain why I liked it.
 
1) No cutscenes. Everything that happens in the game is from your own perspective. YOU ARE THE ONE FREE MAN.
 
2) Believable characters that you care about. Sure, I didn't much care for Barney, but Alyx, Dr.Kleiner and Dr.Vance have superb voice-actors behind them. 
 
3) Related to point 2, Half Life 2 was probably the first game ever to feature extremely detailed facial animations, in-game. When you look into your compagnions' eyes (mainly Alyx) you can see how they fit into their world and how they feel about the current situation. This feeling is enhanced by the well-written story and the mentioned voice-acting
 
4) I mixes alot of good setting/situation points: 
a) The 'Oh, I'm powerless I need to run away!' moments. The entire Citadel is lit-up just for you and they will come down on you like a hammer.
b) Squad battles! I am not alone in my quest against Combine.
c) The gravity gun. Wherther you are out of ammo and need to kill a dude, or in a puzzle that you need to solve, the gravity gun is fun to play with.
d) The 'Oh shit I just did something totally awesome/scary-go-nuts and I cannot tell a soul about it!' moments. Loads of these quiet moments where you crawl through places nobody has dared to for years.
e) Uber-gravity gun! 

5) Implied story. Alot of the story that will be told is not told to you directly, but inferred by the environment around you. Examples of this are Ravenholm (we don't go there anymore) and the apartments that you go through in City 17. 
 
6) Related to point 5, all the skipable dialogue. Mainly all the Dr.Breen stuff on tvs and radios you can find, in which he tries to justify his collaboration with the enemy to a broken people, who see him as the enemy.
 
7) Zombies!
 
There are tons of reason to like Half Life 2. It may not be the best game or best produced game ever, but it's the one FPS that will stick with me forever.. untill Half Life 3 comes out in 2020.

I, just, couldn't read, that. You, need to, learn how to, use, less, commas.

The Half-Life universe is the most interesting and atmospheric universe in gaming for me. I've said before. I'd buy HL2:e3 if it was just a picture book. I just want to know more about the world.

I thought your post was a good read, I have thought that Half Life 2 was overrated for a while now, and not just because the interesting premise doesn't really lead anywhere.  What I enjoyed most about the original Half-Life was the enemy A.I and the intense firefights, this seemed to go backwards in the sequel in favor of gimmicky physics stuff.  Also whenever you pick up an object either by hand or with the Gravity Gun it feels as though the game is glitching up - like a developer's console has just been switched on or something.  Yes I also found the puzzles obtuse, and the characters uninteresting.  
 
Anyway I think that delivering a coherent and interesting narrative in first person is difficult, but probably the best example of how to do it is in Condemned: Criminal Origins by Monolith, which hardly ever even rates a mention, while Half-Life 2 is still showered with praise.  I think part of that is due to the fact that for a long time Half-Life 1 & 2 were PC exclusives, something that PC fans liked to stick in the face of console owners.  
 
I have to say too that I hate it when developers think that need to set up a elaborate story (which is almost always painfully mediocre and just has to be suffered through) for every single fps shooter, I don't mind something like Quake 1 that just dumps you into a surreal world and lets you get to it.

"i hate the entire Half-Life series.  *raises flame shield* "

"I think it's overrated... "

 The story and characters are boring and uninteresting, there's next to no internal logic, the engine that was built for this game isn't utilized very well, and it's just overall not that fantastic.

Half-Life 2 is my favourite game. I played it when The Orange Box was released, so please let me assure you it isn't a "You had to be there" thing. It's just incredibly well-designed; nothing is thrown in, the alien and world design is simply amazing, it tells a story without trying to be a movie, has characters that feel real, encorporates puzzles with action very well, and in my opinion is incredibly atmospheric. To be honest, you sound like someone who went in to Half-Life 2 wanting to hate it.

I feel the "largely unexplained, poorly told" story is a major milestone in immersive storytelling because you are experiencing the world completely from the perspective of Gordon, who does not completely understand what's going on, therefore, neither does the player. That's what immersion is, atleast in my mind.

" @Darknezz said:

 The story and characters are boring and uninteresting, there's next to no internal logic, the engine that was built for this game isn't utilized very well, and it's just overall not that fantastic.

I stopped reading there. You sir are a moron... "

It seems like the OP is criticizing non-existant flaws. Badly-utilised engine? Name one thing that the engine DIDN'T do.

Half-life 2 is a well-thought out, well made experience.  It's fun to play and its universe is compelling.
 
To answer some of your plot concerns:  the G-man is supposed to be a mystery.  In fact, he's supposed to be the thread that ties everything together.  He's not a "wizard", but he's an interdimensional being who drafts Gordon to be some kind of soldier, mercenary, or something for his organization.  His motivations are unclear, but the fact that he can essentially pluck Gordon out of the world and put him into an endless sleep make him very powerful and feared, and the mystery that much more compelling.
 
The Combine are an interdimensional empire.  They utilized the portal storms caused by the resonance cascade and the death of Nihilanth in order to cross to Earth. There they defeated the combined armies of Earth in 7-hours, began assimilating the people, and started draining the Earth of its resources.  All of the combine foot soldiers you fight in Half-life 2 are humans who volunteered to join the combine for a greater standard of living, and as they go higher in rank, they become less and less human since they have to receive several types of implants to become part of the Overwatch.  The Combine's goal is not to oppress humanity forever, but to get as much out of them as possible: as many soldiers, and technology, as they can before humanity slowly goes extinct due to the suppression field.  The only actual "combine" you see during the games are the advisors.  Striders and dropships and gunships are all part-biological, part-mechanical, presumably the only things left of a long dead race.
 
It's all supposed to feel futile, that's the point.  Between G-man and the Combine, Gordon is pretty much screwed from the start.  That's what makes it so rewarding when Gordon breaks into the citadel, gets the dark energy-infused gravity gun, and blows that bitch up.  And that's what makes it awesome when the vortigaunts help break G-mans control over him.  Also, the City 17 citadel being the main Combine hub on Earth, it was connected to the others, so when it went up, the other ones also got either shut down or blown up. I can't remember which right now.
 
Anyways, about the characters.  Yeah, they aren't super-compelling or anything.  You'd be hard pressed to find many characters in FPS games that are.  People like Alyx because the facial expression on the characters was mind-blowing at the time Half-life 2 came out, and she was really easy to sympathize and connect with in a way that no game character really was back then.  I thought she made a very good partner in the episodes, because she didn't feel useless while still letting the player drive most of the action.
 
The thing that sticks with me most about the Half-life series is really just the general tone of it all.  You aren't a super soldier, you're a scientist, yet you're being hailed as the messiah of the earth.  The forces you're up against comprise of beings that transcend space and time and stretch all the way into other UNIVERSES.  The fact that humanity is getting destroyed by the Combine, so slowly yet so deliberately, and when the Combine is done, the only humans that remain probably won't look or feel human at all.  The subtlety of it all.  The fact that there are large story elements that you can miss if you aren't paying close attention, like the fact that the resistance actually struck a deal with the G-man for your services, hence why everyone was expecting you to show up.    So much of the story is told in the subtext, and the fact that it's set in an interesting sci-fi universe and I'm a huge sci-fi nerd just make it awesome for me to experience.

You're judging a 2004 game by 2010 standards.  This is only happening at all because the game was so good in the first place that it's still relevant today.
 
It was (and still is) an outstanding shooter, with one of the most well-crafted FPS campaigns of all time (there is a ton of variety throughout that campaign and you can see the effect that had later on games like CoD4).  Half Life 2 an extremely competent follow up to one of the most important games of all time.  Half Life showed the industry how you should tell a story in a video game world (some listened, some didn't) - Half Life 2 showed the industry how you create a physical world that the player can interact with.  You can't just dismiss the physics in HL2 as a few simple puzzles; it was a revolution for how FPS games were made.  The puzzles -and even the gravity gun- only existed to highlight what they had built.
 
Look at how games were made before Half Life 2 and after Half Life 2, and you'll see why it gets the praise it does.  HL2 was as important and influential as Half Life, which is saying a lot.

"5) Implied story. Alot of the story that will be told is not told to you directly, but inferred by the environment around you. Examples of this are Ravenholm (we don't go there anymore) and the apartments that you go through in City 17."

Remember that games are measured by the time they come out.
 
At the time, Half-Life 2 was superior to everything in the genre. 
 
Just like Pac Man was when it came out.

"

WALL OF TEXT!

 
Anyway, put it this way, if half life 2 hadn't been developed, you wouldn't have the Left 4 Dead or Left 4 Dead 2. Hey, what about Team Fortress 2 or Counter-Strike Source??? I think its an amazing game in that respect and we owe it that one :P

"

I just remember HL2 being a rollercoaster ride. The only thing I can compare HL2 is to Modern Warfare 1&2 in the way the game progresses with increasing intensity. I love these kinds of games - they're as much of an awesome ride as they are games. Am I making sense here?

If I reviewed a game using a list of flaws that would make said game a failure, and yet none of those flaws were actually in the game, that would make me a fucking idiot. 
 
Can't imagine who would do such a thing...

Yes, HL2 sucks..on the xbox 360 and even more on the PS3.
 
On the PC? Best video-game ever created. And if you do not understand why it is the best game ever you are just dumb. Dumb moron.
Going in NOW and expecting ''Halo : The good edition'' might also disappoint you because HL2 was not made for retards.

 But what I'm not understanding here is why people feel like they're part of some great story, when the story in the games is confusing, largely unexplained, poorly told, the characters are literally just there to spit out pointless dialogue that does nothing to advance the story, and all the game does is tell the player to go from point A to point B, killing a bunch of nameless, faceless soldiers, solve an extremely easy and half-assed puzzle every once in a while (almost invariably involving "pick up X object, move to Y location, puzzle done), and the only thing really good about the game is how well the shooting works.

Ya know, "nameless, faceless soldiers"! Yep. Did this guy play the game with his eyes and ears closed?

Blasphemy. The suit has flashlight, speed mode and stuff. Also you use a gravity gun, the gameplay is very well balanced, the story is told in a first person mode and every part of the level was tested to perfection.

Part of me agrees with the  poster who said "gravity gun".  This was my first reaction to the title question.  When I first played the game (in probably 2008, orange box), the gravity gun made my jaw drop.  It just works so well.  It allows so much versatility in gameplay (see achievements such as "The One Free Bullet").  This was the first game in my mind to have a physics system that worked incredibly well.  It was obviously developed around and for the grav fun first and foremost.  It also adds to the experience that you get a teaser of the Super Grav Gun.  Just a tease to make the end of the game stand out with a feeling of absolute power.
 
It's not just the grav gun though.  The game is so well polished that, in combination with the art style, it makes a shooter that still looks good today. AND -  Doesn't use the engine well?  Seriously?   
 
Mods support is another reason it's highly praised for sure.  The company that made the game is one of the biggest supporters of FUN in the industry.  I had fun playing this game, and I had fun playing HL1 (probably also in 2008).  Someone mentioned sound quality.  This is one of the features that you might overlook, but sounds really increase the satisfaction of performing actions in a game world.  Look at the sounds of the boat and the car in HL2.  Those for me increase the satisfaction of operating the vehicles.
 
I have been itching to play HL2 again.  Just so many other games to play....

The only thing I didn't like about Half Life 2 was that every LOADING... time I was getting into LOADING... the game and into LOADING... the moment, the game would LOADING... load.  No matter how fast the loads are, it's still stopping gameplay which I don't find to be acceptable.

" - Story told exclusively interactively in a cleverly designed world 
- It does not try to imitate a movie, trying to play the strengths of the medium 
- believable, well written characters that are not fanservice 
- Original gameplay ideas and lots of polish, trying to engage the player in many different activities instead of waves and waves of enemies 
- excellent art design, especially early in the game   
- excellent and satisfying sound design

I personally thought Half life 2 was one of the best games ever made.  Almost every moment in the game, I was saying "wow!".  I've haven't been blow away by a game like that since I played Ocarina of Time.  I think for me, it was the character models with the great voice acting (I still can't believe that companies have yet to catch up with Valve in the lip-synching department), fun game mechanics (firing a gun felt so right) and the (for the time) innovative physics puzzles made it a unforgettable game.  Not to mention that it had some of the best action moments ever with the battle with the Striders, the battle with the gun-ship on the bridge, and the fire fight you had in the jail.  The game also seemed like the definition of "good level design".  The game is linear--no arguing that.  But it never felt like the game was making you go only one direction.  For the most part, I was just going the way I wanted and was surprised that it ended being the right way to go.  Loved it.

I agree, for the most part, with the original poster.  Even more so now that it almost looks as if Episode 3 is the second greatest example of vaporware ever.

After I first finished Half-Life 2, I remember absolutely loving it.  The whole experience was something I could remember fondly, excluding the time I spent in Ravenholm which is best not thought about too much.  Replaying it recently, though, I found myself constantly grasping for straws when trying to remember why I had enjoyed it so much.  I was constantly saying to myself "Well, when I get out of the stupid boat ride, I'll be at the good part."  "Okay, when I get through Ravenholm..."  "Once I get through the stupid buggy driving section, for sure!"  "Well, once I get past the antlions" "When I make it past Nova Prospekt, maybe?" 
And for some reason, thinking back on it now, it still seems fun.  Playing that game is absolutely infuriating to me, but for some reason I can't bring myself to hate it unless I'm actively playing it at the time.

the campaign is actually good other than say idk halo or mw1/2

" Half Life 2 was one of the greatest games for me I've played in the last 10 years. I can't name one specific reason for it, it's just how I felt after I finished it and still do as to this day. I'll try to get some points across to explain why I liked it.  1) No cutscenes. Everything that happens in the game is from your own perspective. YOU ARE THE ONE FREE MAN.  2) Believable characters that you care about. Sure, I didn't much care for Barney, but Alyx, Dr.Kleiner and Dr.Vance have superb voice-actors behind them.   3) Related to point 2, Half Life 2 was probably the first game ever to feature extremely detailed facial animations, in-game. When you look into your compagnions' eyes (mainly Alyx) you can see how they fit into their world and how they feel about the current situation. This feeling is enhanced by the well-written story and the mentioned voice-acting  4) I mixes alot of good setting/situation points:  a) The 'Oh, I'm powerless I need to run away!' moments. The entire Citadel is lit-up just for you and they will come down on you like a hammer.b) Squad battles! I am not alone in my quest against Combine. c) The gravity gun. Wherther you are out of ammo and need to kill a dude, or in a puzzle that you need to solve, the gravity gun is fun to play with. d) The 'Oh shit I just did something totally awesome/scary-go-nuts and I cannot tell a soul about it!' moments. Loads of these quiet moments where you crawl through places nobody has dared to for years.e) Uber-gravity gun!  5) Implied story. Alot of the story that will be told is not told to you directly, but inferred by the environment around you. Examples of this are Ravenholm (we don't go there anymore) and the apartments that you go through in City 17.   6) Related to point 5, all the skipable dialogue. Mainly all the Dr.Breen stuff on tvs and radios you can find, in which he tries to justify his collaboration with the enemy to a broken people, who see him as the enemy. 7) Zombies!  There are tons of reason to like Half Life 2. It may not be the best game or best produced game ever, but it's the one FPS that will stick with me forever.. untill Half Life 3 comes out in 2020. "

I only got to play Orange Box earlier this year after blowing through HL1 just before.
 
I didn't really enjoy HL1 much, just blowing through it for the story, but Half-Life 2 was incredible. I played a 2004 game in 2010 and it's pretty much the best FPS i've played. For the longest time i'd been wondering what all the love was for HL and Valve, but I was converted.
 
The silent protagonist thing was annoying at times, and I felt that some levels seemed to drag on for too long, but the overall experience is phenomenal.

I love Half Life 2, so you can piss off now :)

"

WALL OF TEXT!

 
Anyway, put it this way, if half life 2 hadn't been developed, you wouldn't have the Left 4 Dead or Left 4 Dead 2. Hey, what about Team Fortress 2 or Counter-Strike Source??? I think its an amazing game in that respect and we owe it that one :P

"