This being out of the way, let me explain why (just in case you haven't read one of the million reviews that already say so). Some time ago, I wrote a blog post with my good friend Fabien Royer about what we consider to be the seven deadly sins of modern game design. We pointed out that even great games such as Gears of War or Oblivion always had at least one of these flaws. So how does Bioshock measure up against those (arbitrary) criteria?
Checkpoints: Bioshock has checkpoints, but they're not your (big) daddy's old checkpoints. They are non-destructive checkpoints. They don't affect your progression, they just respawn you at some specific point. This is absolutely brilliant as it never punishes the player for trying something new. In this game, you'll never have to redo the same boss fight a thousand times because you died a millisecond before the boss: the damage you inflicted persists even if you die. It all plays beautifully as the game's difficulty is very well-balanced (if a little on the easy side, even on the maximum difficulty level). While it's true that you could take down a Big Daddy with just the wrench if you're patient enough, it will take forever and you won't have as much fun as if you waited for him to step into gasoline and set him on fire after tricking security turrets into attacking him. I love how they put the responsibility of how fun the game is in the hands of the gamer: the game is just as fun as you make it.
Oh, and on top of that, you can save whenever you want.
So, yes, perfect and innovative checkpoint system. Bioshock, one point and a half for brilliant innovation.
Boss fights: see above. Thanks to the brilliant checkpoint system, there is no boss that you have to fight more than once. You may die several times in the process of defeating him, but you don't have to redo the same thing over and over again. The great game mechanics also encourage the player to try a different approach if they fail: bosses don't have a single weak point, they are just stronger or weaker for different types of attacks, and there is always a large number of ways to kill them. Bioshock, one more point.
Mini-games: ouch, Bioshock almost lost a point here. There *is* a mini-game (hacking) and like most mini-games, it's disconnected from the main gameplay and not much fun at all. But it redeems itself by being entirely optional. If you don't like it, you have plenty of options to get the same results: you can buy your way around it, you can use an auto-hack tool that you built or found in the game or you can just do something else entirely and not attempt the hack (destroying the security device, paying full-price in vending machines, etc.). So, well, half a point here.
Cut-scenes and dialogue: the narration in Bioshock is absolutely brilliant both in contents and form. You learn about the background plot by finding recordings as in-game objects. These recordings play while you can continue playing so they never disrupt the rhythm of the game. There are also a few cut-scenes during which you can look around a la Half-Life, so it's not full interactivity but they are at least very well-integrated into the game. I never felt the urge to skip them so I don't even know if it's possible, but that's a good point: you don't want to skip them. And you don't have to watch them more than once because again of the brilliant checkpoint system. One point.
Reload times: the loading times in Bioshock are not especially short nor especially long, but again, the checkpoint system makes it so you won't have to endure them except when going to a new level or reloading a saved game (which you'll almost never do). So nothing exceptional here, but the generally great design of the gameplay makes it a minor inconvenience. One point.
Camera: it's an FPS, so you are and manage your own camera. One point. Oh, but wait, your senses in Bioshock actually go well beyond sight. The audio and noise localization is the best I've seen in any game. You'll usually hear the enemies before you see them, which really creates an incredibly immersive experience. You need 5:1 to enjoy the game at its full potential by the way. One point and a half.
Control scheme: it's ok. I've inadvertently injected myself EVE more than once because of some button overloading but it's nothing terrible. Half a point.
So all in all, where are we? Seven point! We have perfection! OK, I cheated a little and gave more than one point here, less than one there. But that's actually how this game is: it is so brilliantly put together that everything fits and the very few minor weaknesses it may have are made completely irrelevant by other design decisions. The checkpoint system is an example of a very simple innovation that is much farther reaching than it may appear at first.
This game is pure joy, it is art, and if you're old enough (it is also very violent, be warned) you'd do yourself and the industry a great favor by buying it. This game must be a hit because we want many others to imitate it...