What do you think makes a game re-playable?

Now im not making this a poll because lets face it, there are so many opinions out there. 
 
For me it is mostly for the story. 
It has to be good enough that i want to experience it again and again, year after year. 
Games i will play lots of time's include: 
 
Persona 4 
FF9 
FF10 
Mass Effect 
Mass Effect 2 
Dragon Age  
 
What are some of yours as well? 

The experience in itself. If it's awesome, it is worth repeating.

Jiggies, lots and lots of Jiggies.

leveling up, creating a world where going back in means more fun loot, or The kind of "New Game +" that just breaks the game and makes it fun to blow through it.

I don't replay games for their story. Once i know the story, there's no reason for me to go through it again. To back up that fact ; if I ever replay a western RPG like ME2, i'll just fast forward the conversations. 
 
What makes a game replayable for me is the overall experience and gameplay. God of War, Alan Wake, Red Dead, Brutal Legend, Batman AA, the Resident Evils... and so on.

Choices I guess? I don't actually replay games unless I lose the save games, but different choices allowing you to have a different experience than your first play through would make it more enjoyable.

Nothing

"  Persona 4 FF9 FF10  "

Games with a good story, fun game play and ones with choices (so that when you replay you just about get a new experience even if in the end it's the same game)

Being able to develop your character after the story is over. A continuation of the story, maybe...I'm saying this because I was going to say achievements! But figured there was a reason I went back to AC2 after completing it. Other than that the only thing that will do it for me is nostalgia. I really enjoy replaying Zelda: OoT.

Good gameplay.

First it's probably the gameplay. If the game is fun to play, then I would most likely want to play it again. For example Jedi Academy. The story is shit, but fighting with lightsabers is fun as hell. Also no fight plays the same each time. Even if you reload a save something different can happen. So, something linear and also scripted like COD might not fair as well for a second play through. The second thing is how customizable the game is. Does the story have alternate paths e.g. Bioware games. How much can you change your character. That kind of stuff will make me play again as a different style character. 

The story is the main one for me. Also New Game+ features such as more leveling (if it's that kind of game), new weapons, new armor, if there are multiple endings or multiple scenarios, different ways to play like classes or a good/bad karma thing...

oh also i would say that for me nostalgia for a game also helps the re-playability 

" The story is the main one for me. Also New Game+ features such as more leveling (if it's that kind of game), new weapons, new armor, if there are multiple endings or multiple scenarios, different ways to play like classes or a good/bad karma thing... "

The quest to improve on my current skill level. 
Games like Burnout Paradise and Super Meat Boy, where they make it easy and fun to try and improve your skill. 
Same goes for Mirror's Edge. 
That's why Platformers are my favorite genre. 
My opponent is my previous self. 
 
LittleBigPlanet's "Campaign" got a lot of play out of me. Beat the level,  collect all the goodies, now beat the level again, without dying. It's an added challenge the masochist in me just can't refuse.

1.Non-linear RPGs and dungeon crawlers. I want to play the game again, but differently. 
Perfect examples are Demon's Souls, Heavy Rain and, obviously, Mass Effect 1&2.
2.Games with awesome multiplayer. I just want to play it :) StarCraft II and Killzone 2/CoD 4 for me.
3.Games with collectibles and not so much focus on the story (let's say God of War and inFamous).
 
I don't really like replaying linear games (especially RPGs). I know, for example, that I'll propably never
want to play Persona 4 again ("This guy dies. That guy gots kidnapped. This person's the killer"), even
though along with StarCraft II it's the best fucking game I've ever played.

Variety. Sure, sometimes the story can be awesome enough, but I generally don't have time to re-play those games anyway.

The lack of flaws is what makes a game very replayable. Look at everything on gameranking's top 90% list, it stood the test of reviewers and time, I can replay those games over and over. It got to AAA 90% status because it doesn't have any flaws and it's like all strength. You will find games like lower than 80% to be not very replayable at all. That's my take on it anyways.  
 
I don't mind replaying half life 2 again.

Gameplay.
 
I still play NES versions of Contra and Tetris on long road trips because those games stand the test of time.

new things in the game and different experiences. the only game i restarted this generation was resident evil 5. it was because of getting infinite ammo for different guns. it was fun to kill zombies again with a different weapon. i think that was really this generation that i did that.

Story is a major factor, but its not everything.  I've gone back and beaten Metorid Prime for Nintendo Gamecube 3 times now, and I don't find much story in it.

I guess it was just for the fun and simple gameplay that comes from killing stuff and collecting things. 
 
So, what makes a game replayable for me is probably good, reliable gameplay, that doesn't get to overly repetitive.

I'm going to go with gameplay is important but I also love when a game gives me reason to play it more times. The best game I can think of that did this is the aforementioned Resident Evil 5. I played through all of that games modes. Also Lost Planet 2. Thats right! I find that game so addicting and there is almost always more things to unlock. Another game I played through three times was Tales of Vesperia. For a game that long, that is something I never do. I put over 200 hours into that game and I skipped the cutscenes as best I could by mashing the A button(there was no skip option). It was just that much fun to play.

For me it's a bit of everything layered on top of a large cake made up of leveling. A big scooping of customization ice cream fills me up (:

I usually only replay a game if the narrative has different ways of playing out. My free time is limited, so I'd rather use it playing new games than stuff I've already finished.

Good story, great gameplay, and a variety within the game.

" First it's probably the gameplay. If the game is fun to play, then I would most likely want to play it again. For example Jedi Academy. The story is shit, but fighting with lightsabers is fun as hell. Also no fight plays the same each time. Even if you reload a save something different can happen. So, something linear and also scripted like COD might not fair as well for a second play through. The second thing is how customizable the game is. Does the story have alternate paths e.g. Bioware games. How much can you change your character. That kind of stuff will make me play again as a different style character.  "

Fun.

F is for Fire that burns down this whole town 
U is for URANIUM...BOMS
N is for NO SURVIVOR when you're...

It's all about the gameplay for me. I love games that are relatively short but challenging. I'm creeping up on that achievement for beating Mega Man 9 thirty times...

What makes a game re-playable is one of two things.
 
1. The game has to have something like Dragon Age where you can go back and have a completely different experience the second, third, or even fourth time and beyond 
 
or 
 
2. The game has to be one of those games that is sooo amazing that you can go back and beat it and still have an amazing experience. For example, I play Ocarina of Time every few years because it's still such an amazing game, and one of my favorites. 
 
Other than that I dont really see there being much other reason to replay games unless you dont have the money to get new ones. Most of the time I will beat a great single player game like Uncharted 2 and then never play it again unless I go back years later to play it. Keep in mind I'm not really considering any multiplayer part of a game, only the single player.

The experience. I mean experiencing the game over again. That means gameplay, story, etc. all rolled up into one. I've played TFU about 10 times. I expect to continue playing it for many more times to go.
 
Sometimes, though, it's just for achievements. Like MW2, I beat the game on Veteran (hard as shit, but still not as hard as blops, by a longshot), then went back on Easy to finish picking up all the intel.

Either a really good story/characters/universe/etc or the complete lack of a story.