I'm not a Halo hater, nor am I a member of the "Halo is overrated" camp. I just can't get into the series, but virtually have nothing against it. Ever since Halo 3 was released, the Halo contains the most polarizing opinions, than any other popular or critically acclaim game and game franchise out there. Even amongst the fanbase, there's a bit of a breaker base in opinions, especially regarding the single-player and storytelling of each game in the series. Despite it's large fanbase, it has a large group of people who hate it and people who just think it's overrated. It gets as much backlash, as much as it gets praise. So the Halo series has been controversial among the gaming community in this regard, while I have been neutral towards this series as I couldn't get into it and haven't played enough the games to begin with.

Despite all of that, I can't say that I know the exact reasons why people like Halo. I like to understand it's appeal by asking the fans this question: What makes the Halo series so great for you? What does it do to make it enjoyable for you?

CTF on Warsong Gulch. Why the heck did I pay full price for that game when all the fun was to be had in the demo?

Interesting universe, great music, a developer that responds to fan feedback, and multiplayer that requires more than twitch aiming.

If it's not for you then it's not for you. Everyone has different tastes and it boggles the mind that people can't grasp this concept.

Originally because it was the best looking thing ever thanks to bumpmapping, the Warthog physics, the way the grenades handled, and the AI. Also the enemy and weapon variety and "combat puzzle" and of course the sheer size of the environments.  Then there was Blood Gulch AKA coagulation. Probably as good as facing worlds if not better.
 
Plus like people have stated before, the multiplayer is very tactical and you can't win just by unloading a few rounds into someones kneecaps. Also one of the first games I ever played that had a melee and grenade button. Previous shooters made you switch to those before you could use them. Making them fucking useless most of the time.
 
The partical effects in the halo series have always been really good too. And shit is always full on bananas and shit is usually exploding all over the place at all times. The framerate stays playable at the most chaotic of times and I'd call that important. That's off the top of my head. I could go on forever about why halo is so great.

" If it's not for you then it's not for you. Everyone has different tastes and it boggles the mind that people can't grasp this concept. "

The thing that I think Bungie does really well and that always hits me when I play a new Halo game is how they balance smoothness and weight - if that makes any sense.
 
I'm not a crazy fan, I usually get bored of it after a while and move on until a new one comes out, but something about the way they do it just feels good to me.

Balanced multiplayer and the custom multiplayer games that no other console game can even come close too.

The moments where the music and gameplay sync up perfectly. The laid back multiplayer that can be majorly enjoyed with friends.

" Interesting universe, great music, a developer that responds to fan feedback, and multiplayer that requires more than twitch aiming. "

" @Godites: I'm not saying that you are being smartass or whatever. I'm merely stating that people have different tastes. Many people like COD but I on the other hand don't give two shits for that game anymore because the multiplayer doesn't attract me at all like it used to. People have already given their reason as to why they like Halo and if you don't think they are interesting reasons then that's your right because you're a different person. "

It's the best split-screen game on current-gen consoles. The campaigns are usually pretty good too.

" Interesting universe, great music, a developer that responds to fan feedback, and multiplayer that requires more than twitch aiming. "

It's fun.  

" It's fun.   "

" Interesting universe, great music, a developer that responds to fan feedback, and multiplayer that requires more than twitch aiming. "

Halo has that Perfect Dark/Timesplitters feel to me, which means good stuff.
 
The modes are robust, the weapons are all great (especially in Reach the balance feels near perfect) and the singleplayer is always a real blast to experience.
Also, the recent addition of Firefight in ODST and Reach has only made me love Halo more, the game just feels so good that you can keep playing Firefight and always have fun doing it.

It's the weapons, which have some interesting effects.  They aren't just all hitscan, and some do crazy things.
It's the vehicles in both single player and multiplayer.  Again, they're varied and some are just crazy.
It's the enemy AI, which is good enough really.
It's the music which is often awesome.
It's the sci fi theme, which doesn't try to be edgy cool like so many games today.
It's the level design, which is better than most FPS today.
 
Those are the big things for me, I think.
 
Oh, and the recharging health actually makes sense in this one, because it's shields.

Halo: CE was the first FPS that was any good on consoles. I bought an XBOX, Halo and an extra controller and went directly into co-op campaign and it was a completely new experience. Sure I had played some Q3A online, but I had never played a co-op FPS game. It was like Ikari Warriors (played a lot of that in co-op on the NES) all over again, but better. 
 
I think the enemies are fun to play against because the elites require some strategy since they have shields. Also the AI was much better than anything else at the time. I liked the weapons, the slower pacing compared to PC FPS games, the music, the atmosphere and Cortana. I just fell in love with it and since then I've enjoyed it, but it's never been like Halo: CE ever again. Next to Half-Life 2 it's the best FPS game ever IMO.