I just beat Bioshock 2 and I enjoyed it much more than the first one. I don't know why, when people mention graphics and gameplay, I care but not enough to really /care/ if that makes sense. When I look at a game, the one thing I really look for is story. It may be a poor reason to buy games, and I won't buy a game with horrid gameplay or anything, but Bioshock 2 definitely didn't have horrible anything.
The first game just didn't catch my interest, I played it, I grew tired of it, I stopped playing it. The second one hooked me in right from the get go, I loved the story and the characters and everything about it. Bigsisters are freaking terrifying and the game still has plenty of parts that were eerie and creepy and downright scary at points.
Even if it's graphics aren't as good as the first (which I honestly didn't even notice, so I think it's just nitpicking) the second one tells a better story in my opinion. The first one felt like "Oh snap, these people don't like you. If you'd kindly kill them we'll be on our way." At least it felt that way to me. In the second one, I felt compelled to do what I did, I felt like it meant something, I felt like the enemies were in my way and I needed to get past them. For a reason. Not just because they were there.
I don't know if it's just me, but that's most definitely my take on it and I'm sticking to it.