Whilst not having completed the sequel, I feel the general consensus is that is was just simply not as memorable as the first, mainly in terms of storyline, as I feel the gameplay is generally consistent with the first.
I thought it topped the original in a number of ways. Being able to simultaneously wield plasmids and a firearm was a huge improvement and helped keep the gameplay from getting stale. I also liked the premise just as much, but I don't think it was told quite as well.
I'll just throw the rest in spoiler tags for safe measure (story content):
Spoiler
As a replacement for Andrew Ryan, Sophia Lamb failed miserably. However, she was still an interesting antagonist and I thought it was great how much her views differed from Ryan's, despite being on the same "bad" side. Much like Ryan and Fontaine, it reinforced the "every man for himself" aspect that was the foundation of Rapture, and chief contributor to its demise.
I also thought that Delta and Eleonor's relationship was done extremely well, possibly being the sole story element that was fleshed out as well as its predecessor.
I also thought that Delta and Eleonor's relationship was done extremely well, possibly being the sole story element that was fleshed out as well as its predecessor.
Other than that, the environments didn't leave a lasting impression like the first time around. They were still detailed and fell in line perfectly with the first game, so at the very least 2K Marin didn't butcher continuity. Overall BioShock 2 was a good sequel to a game that really didn't need one.
Edit- I realized I said a number of ways and went on to list just one. Oh well.