So I'm playing Bioshock 2 right now, and I'm actually quite pleased.
I'd heard all sorts of disparaging things about the sequel, and some of them aren't hard to see. The story is kind of a rambling reversal of the entire Bioshock storyline (creepy female demagog vs. cold authoritarian tyrant) and the game didn't feel fresh at all the moment I fired it up. The first few levels string out the feature addition a little too slowly, which might be why people found it so bland to start.
But once you get into things it really picks up. There's so much more to do it feels like than there was in Bioshock 1. There's more Plasmids and Tonics available early on, collecting ADAM can be a lot more involved if you want it to be, you're working with more weapons and ammo than you were in Bioshock, right from the outset. Research takes much longer and is kind of more fun to do. All in all the game is just meatier than Bioshock was by several orders of magnitude. I suspect it may be a smaller game overall, but it packs way more content into each area. Playing it with all the newbie controls turned off too has made it feel like a real scavenging/survival game, like a S.T.A.L.K.E.R-lite (ok, maybe extremely light.)
It really does feel like a real successor in terms of mechanics, even if the story already feels old by the time your an hour into it. I guess there's people who didn't feel they got enough of Rapture in Bioshock 1, and then there's people who felt like they got plenty.
Playing through this though is making my want Bioshock: Infinite.