How long am I going to play this game for? Is it going to pull me back in at some point in the future? How much thought will I put into playing this game? (The more the better)
Replayability is usually determined by the level of customization offered. Being able to play the game in a different way usually changes a mediocre experience into a fantastic one.
A good example of a game I come back to often is Civilization 4. It has lots of fantastic mods, is fairly open ended and allows you to play the game in many different ways without forcing you into a single strategy or objective. RPGs usually have a fairly high replay value as well.