Game replayability.

[BumblebeeCody]

Depends on the type of gamer too.

Some games like Heavy Rain give you option the completely change how you played your game first time through. Mass Effect has a pretty direct version of that (good or bad).

Some games like Resident Evil, Mario, Zelda, Sonic (and lots more) have a large speed running community. Completing the game in the fastest time possible is a fun challenge.

Games that primarily based on multiplayer/versing an opponent like Fighting Games can stand the test of time. Games like Super Turbo are still being played today at high level tournaments 20+ years on. But it's because the mechanics don't age so the replay-ability is endless.

As with this generation (Xbox 360 especially) there are these achievements/trophies so it gives you a reason to go back and play through the game on hard/without shooting someone/without dying etc