The first, solid, details have emerged from the GameInformer article about Bioshock 2. The information has been confirmed by numerous users on Neogaf, so it is safe to assume it is true. Here is the summary of what was in the article.

* Summary: You’re a Big Daddy, and you get to use the drill and rivet gun. And you can use plasmids too. You can take little sisters from other Big Daddies and take them around Rapture to collect ADAM like you see happening in BioShock 1.

* It’s 10 years since the first game. The Big Sister returns to rapture, bringing back the Little Sisters with her. You play as the original Big Daddy. You can use the drill, the rivet gun, and you can still use plasmids.

* Little Sisters have been given a new look to be less creepy, as you’re seeing them from the eyes of their protectors. You don’t get your own Little Sister from the start, as you’re a renegade Big Daddy. So you fight other Big Daddies to get your own sister. You can still harvest them, but the other option has been changed to “adopt” instead of rescue.
* If you adopt the sister, she’ll ride on your shoulders, and you can go around Rapture looking for corpses for her to collect ADAM from. When your sister goes to harvest ADAM, a wave of Splicers will come and start attacking to try and harvest her. You have to hold them off until she finishes harvesting. You gain some ADAM from this, but not enough to tide you through the game. You can permanently rescue them, but the article doesn’t say how.

* The Big Sister is one of the surviving little sisters from the last game. She’s the new ruler of Rapture, and is the main antagonist of the game. With each Big Daddy you kill, the game lets you know how close you are to incurring the wrath of the Big Sister. When you do, the little sister you’re with will sing a song about how the Big Sister doesn’t want you playing with her, then you get jumped.

* The Big Daddy drill takes the place of the wrench. Can spin, but overheats so you can’t run it constantly. You can do the Big Daddy charging shoulder bash. You can heal your hacked turrets/camera bots. Incinerate upgrades let you throw fireballs and shoot a stream of fire. Cyclone trap can be hit with incinerate and become a flaming cyclone trap. Upgrade trees are more diverse to give you more unique options.

* It says they’ll think twice about attacking you on their own, but if they’re in groups they’ll tear your **** up.

* More info on big sister: “The Big Sister will always be a desperate, desperate fight…She can find you wherever you are, so you have to prepare for the fact that the hardest thing in the game is on the scent.”

* She’s fast, stabs you with her ADAM needle, then uses TK to suck in all the stuff in the room as a shield, then launches it at you.

* We get to see how ADAM gets extracted from Little Sisters for distribution.

* You walk around on the sea floor outside of Rapture

* Addendum: Vita Chambers aren’t back in the same way as they were in the first game, but the same mechanic exists. Can be turned off.

* Game is supposed to be scarier than BioShock 1.

* Other info: There is confirmed multiplayer (no details), audio logs are back, and Fontaine Futuristics is a level in the game.