I just got my copy of game informer today, and all I got go tot say is I can't wait for this game to be released.
What I'm writing below is paraphrased from the article I read, so please excuse any mistakes on my part.
Apparently the game starts 10 years after the events of the first game, a little sister who had once escaped rapture has returned, and has pretty much taken over rapture as the new guardian and making sure everything follows the status quo.
Your character is *suspenseful pause* a renegade Big Daddy, but not just any big daddy the first one ever created, complete with the iconic drill arm and rivet gun, and naturally enough your character has access to various tonics and plasmids, which now have various uses beyond what you had in the first game,
when you level up plasmids this time around different effects happen with each upgrade, say for example you have the pyrokenesis plasmid the first tier will just set an enemy on fire, the second tier you can charge up a fireball for extra damage, and one of the last tiers turns it into a flamethrower type of weapon.
Now seeing as your character is one of the intimidating big daddies, you might think the game will turn into a cake walk, not so, while as a big daddy you can dispatch one or two splicers with ease, whole groups of them can be rather challenging to overcome, and you still have other big daddies to deal with, since you are a renegade you don't have your own little sister to look after, so whats the logical solution to this problem?
Well, steal one from another big daddy of course! now after taking out the rival big daddy, you come up with a choice not unlike the one from the first game but with a little twist, whereas in the first game you can either harvest them for their ADAM, or save them for a bonus later on, in the sequel the harvest option is still present but you can also "Adopt" the little sister and carry her on you back so she can harvest the ADAM out of the "angels" that will be lying around the areas, but doing so will attract splicers to come and try to kill the little sister for the ADAM she has with her, where it is our job to keep her safe until she finishes what shes doing, this all goes towards that "moral choice" thing from the first game, she also give you some of the ADAM left over once she gets it all.
Now comes the final aspect of bioshock 2, the Big Sister, now after she goes though the trouble of kidnapping little girls to turn them in to little sisters, and get rapture back on it's feet, you don't think she would be too happy with you coming in and killing off all her big daddies and either killing the little sisters or saving them, fro what I gathered from the article there is a kind of alarm system where if you mess with things too much, she'll come after you, sand she is faster stronger and being once a little sister herself, she can use that needle in the picture above this post to extract ADAM straight into her bloodstream, giving her a much better skill with plasmids than you, so alerting her to your presence isn't a very good thing, obviously you will be able to hold her off or else you wouldn't be able to finish the game, but don't expect to get through those battles unscathed.
Onto the world of Rapture itself,, the Vita-chambers make a return, but they weren't very clear as to what they do this time around, but there is an option to turn them off completely like from the patch for the first game, while exploring more of rapture in this game than you did as Jack from the first, one of the new locations is Fontaine Futuristic's, which was mentioned in the first game several times, The area where little sister go to get the ADAM harvested out of their bodies, which probably won't be to pretty to see when you go there, and also there are time when you get to go outside of Rapture and walk around on the ocean floor, these sequences are normally there to let you relax while you go to your next objective, since it is unlikely you will find Splicers wandering around, although who knows if you'll ever run into the Big Sister out there although that's only my own speculation.
EDIT: Also there will be multi-player, but they haven't said anything on the different modes yet