Spectacular concept and amazing emotional depth.
The story is so well-polished, and everything fits together so neatly (assuming you connect the dots and get most of the audio diaries), but, because Rapture just engulfs you, I was left wanting to know more about how Rapture came to its demise, and what it used to look like at its peak.
Though BioShock is just a game, it creates such a mood and gave me such sympathy for the partially-insane-but-still-know-what-they've-become splicers, but there was something about the Little Sisters that made them seem less human, and so they would have been easier to harvest (though I still saved them)
The only thing that really let the game down was the lack of visual realism in the enemies and cutscenes. From the (DirectX 10) images I saw of the game, the splicers look a little odd close-up, which didn't worry me too much. But what irritated me was the lack of human-resemblance on the non-mutated characters of the game like Andrew Ryan. Their faces seemed far to blocky and pointy to be passable as 'alive', especially when comparing them to what DirectX 10 can do.