Bioshock is a very good game but I could not see what's so great about it.
Here are my points that make the game not worthy of a 9 IMO:
The game is just sooooooo easy - vita chambers make you never to care about your health bar. They should have been checkpoints instead and no other save options and the game would have scored above 9 for me.
Total lack of enemy diversity - shooting at the same uninteresting humans the entire game, wether they crawl on walls or not, just doesn't cut it. It is even more frustrating how these same humans become suddenly much more powerful at some point in the game. Big Daddies are fun, but they ignore you if you do not attack them so when all Little Sisters are collected, there is no need to oppose them. And finally, only one boss fight in the entire game (not counting fights with special Splicers) and it is the final showdown and it is a major disappointment in how easy it is.
Hacking seems ingenius at first, but it quickly becomes rather boring, especially when there's something to hack in every room.
There is a great variety of plasmids, however most of them are utterly useless. They have upgrades too, but instead of getting those through using your plasmids, you just purchase the upgraded version from a machine. That's stupid. Weapon upgrades are much fun though. And speaking of weapons, they are all pretty good in general even though they are quite standard - a revolver, a shotgun, a machine gun, etc.
The story is actually good and the game is very strong at the point where it creates a thick feeling that the game is ending and then all of a sudden there is a quick and rather unexpected turn of events and it seems like it all starts again... which is actually not that positive, because the game becomes monotonous fast after that point. Still events in the game are fun but the story could have been told better than with radio messages which are sometimes hard to follow. However, it is good how those radio messages present bits of many aspects of everyday life in Rapture which definitely serves the atmosphere.
I totally fell in love with level design in the game. Some of the places like the tree garden and Fort Frolic are just marvelous and the idea of these levels is great.
One more plus for Bioshock in the end - the game is rather lenghtly for a FPS which is good.
With all that said, I think Bioshock had a tremendous potential to be the best FPS this year, however, the way it is now, it will probably be blown away by Crysis, Enemy Territory: Quake Wars, etc. Hell, I even found Medal of Honor: Airborne more fun, because it was more challenging. And what about multiplayer?!?!