Even though I didn't believe the hype at first, I really got it after a few minutes of play-time.
Bioshock is set under the sea in a dystopian city full of libertarians. Well at least I saw that coming. Anyway you have regular pyschos come up and try to kill you by any means possible, but the things you should look out for are the Big Daddies. Though the fights are usually optional.
However somehow you are tempted to fight the Big Daddy. Not for the in-game bonuses or even the Xbox 360 achievement, but for the fun and challenge of it. Plus those guys are so scary that you don't won't them walking around anyway.
Plus the story, characters, and environment are so top of it's class. I loved every minute of my time in Rapture. Very nice tourist attraction, not so nice residency.