I played the first hour of bioshock today and wow, was that amazing. What gets you is how tense it is. Around every corner, there could be a splicer, and while they aren't hugely dangerous, the game is able to make you worried that around any corner there could be a splicer and that a splicer getting the drop on you is fatal.
Ofe course the art direction and the world of rapture are great. And the rest of the game. More on it as I play.