A beautiful game - the environment was stunning, you are in an underwater city, Rapture, yet you never actualy play in an area that is completely submerged.
The AI is done very well also, the world "lives" - enemies do not respawn, per se, but they do move all over the city, so once you have cleared an area, other enemies will eventually move into those areas as they go about their business.
The storyline was a little cliche, but they did a great job of keeping it clean and non-confusing. I did not once feel that I was missing something to the story, or was being kept in the dark about the true purpose of Rapture.
The weapon systems is done almost too well - there is a constant stream of ammo and ADAM, you never feel that you don't have enough ammo or ADAM to get through a fight.
Another cleverly done aspect are the new skills/spells you learn throught the game - in one area, you learn to cast a fireball, and that area is full of things to be melted so you really learn to use the skill to its best ability.
The 'hacking' puzzles were almost too easy, but still requred some thought to complete, which was a nice break in the action (for a few seconds anyway!)
All in all a great game both visually and game play wise. It is a little RPG-ish, a little FPS - all gamers will find something to enjoy with Bioshock!