Short version: Bioshock 2 is a game that, at least for me, surpassed the original in just about every possible way. It's a gem that sadly seems to have been more or less forgotten - if you have enjoyed the original at least a little bit, or, like me, completely forgot that a second game exists, get it and finish it now.
Long version: Bioshock 2 just more or less took everything that was wrong with the original Bioshock and fixed it, while retaining everything that has made it so good. It controls better, weapons are more fun, plasmids are more interesting (and upgraded versions are not just the same thing with more damage output, finally), story is pretty damn good... It's just all around a fantastic game. Now my biggest issue with the original Bioshock was that the further you got along the storyline, the more bullet-spongy the enemies become, to the level of utter ridiculousness. Well that's gone in the second game, and good riddance. The stereotypical weapons in the original were replaced by quite a bit more interesting ones, especially after you fully upgrade them, and every single weapon retains usefulness until the very end of the game, including the default meele weapon. Which is a drill. And I pretty much exclusively used that the last 1/4 of the game.
Now playing as a Big Daddy was a brilliant idea from the part of the dev - it retains the thought from the original game, 'you only have little freedom in videogames', and as far as I'm concerned, it implements it in a much more organic fashion. And combat makes quite a bit more sense as well - I mean, I never understood how the guy from the original could defeat everybody seemingly so easily, even after the 'twist'.
And then there are big sisters, overhauled handling of the little ones... The game quite simply was awesome. Why do so few people mention it and how comes the original overshadows it so much?