Bioshock was a finely crafted work of art. The trouble with an excellent story, enriching world and innovating gameplay is that those things together tend to make a "complete"experience. A complete experience does not need a sequel, and worse yet, it does not ALLOW for a sequel. Lightning can't strike twice here, and no one can experience Bioshock for the first time again.
Does that mean it'll be bad? Heck no. It'll be one of the better games of the year, more than likely. But Bioshock was fresh and invigorating in a sea of gross Halo-killers and shooters that thought "Aliens" was an acceptable plot point. Look at a Metacritic or Gamerankings score for Bioshock right now. Almost nothing under a 90% in over 100 reviews, it's one of the top 10 games on Game Rankings. Bioshock 2 will not live up to that.
Adoption? Fairly interesting Multiplayer? Abhorrent. Aquatic sections? Gimmick.
They'll keep the core gameplay, they have a decent budget, they have a decent team. I'll probably pick this up used, and really like it. But there's no reason to be a day 1 customer or pre-order Bioshock 2.