I gotta say, after reading all the comments, I agree that Bioshock 2 is improvement of the first one, that they did offer you little to learn about, and even if the story fits without going against the canon, but it wasn't needed, like Episode I, II and III from Star Wars. But, to be honest, using bioshock and Rapture, there wasn't much else to do. They could try a brand new setting, but probably people would complain about not following the former title, which I would agree.
Maybe a new setting would be refreshing next time, I don't know... Twatshock in the Jetsons setting, but in any case, Twatshock 2 would be pretty much the same, and people will complain the same.
My point is, that complaining about Bioshock 2 being almost the same (which I partly agree, though I don't complain) is like complaining about the cup of coffee you're having now tastes like the one you had yesterday, and you had to pay for both the same.