FightThePower said:
A world can be believeable without being realistic; Metroid Prime and Bioshock were both incredibly immersive games but they aren't very realistic (shooting wasps out of your arms?! Prepostorous!).
There still is a level of realism in those games though. When I saw Rapture, for instance, I was able to go like, "Oh, I see how that might work." I'm pretty sure that's what Pyrode meant when he mentioned some level of realism.
I also hate it when people say, "Oh, realism sucks because I already live in real life." The point of realistic games is to put yourself in situations that you would probably never be in, or to take greater risks in those situations than you normally would (like driving and crashing cars in GTA).