NowGamer has cranked out what is probably the most detailed preview of BioShock 2 I've seen, no doubt due to the fact that the editor toured 2K Marin's offices.
Our demonstration begins in exactly the same manner as the original BioShock game: a character waking up in Rapture, their very existence an unfathomable mystery. As the iconic drill slides into view and Dr Tenenbaum begins speaking through radio static, our collective gasp resonates with the sound of the penny finally dropping. This will be BioShock 2's masterstroke: not simply playing as a Big Daddy, but playing as the very first one, the prototype faster and more formidable than the rest, endowed with plasmid powers and operating under its own free will.
(There are a lot of things that make you special, and the majority of them are pretty critical to the story,) Thomas explains, obscuring the narrative reason for the prototype's enhanced abilities. (Let's just say that they decided to simplify, but I really can't say any more. That's the mystery: you begin as a sentient Big Daddy, and the question is how you came to be like that.)
As the only Big Daddy in Rapture with the power to act autonomously, you have greater freedom than any other resident. Not only will you be able to navigate the city's snaking tunnels and grand plazas, you'll also be free to explore the ocean floor a feature introduced to us in fine style, as a vast circular window slowly cracked and fell in, sending a raging torrent of water crashing towards the screen.