Ive never been a huge fan of Bioshock (greatly enjoyed, but only rented), but this trailers is enough to make me want to buy the game when it comes out. I also just read the (quite in depth) Game Informer article about this same demo, plus commentary from the developers.
NOTE: SPOILERS BELOW. I dont know how much I can spoil about a game two years away from release, but I don't want to upset anyone so quit reading if you want to know absolutly nothing.
Okay (deep breath)
Evidently it's 1912, the city was made in 1900, you play as a guy named "Booker DeWitt", Plasmids are now called "Vigors", Tonics are "Nostrums", the giant robot creatures are called "Alphas", the evil beast at the end is called "him" or "the guardian" and has kept Elizabeth prisoner for 15 years. Elizabeth will be your companion most of the game, but she has a stockholm syndrome type relationship with the creature, so that should make things interesting. The city itself has been split into a civil war between the ruling class (seen in the trailer) and the "Vox Populi". Both factions want Elizabeth, and it's your job to get her out.
(Phew!)
END SPOILERS. Sorry if i offend anyone with this post, but im so frickin pumped about the game that i just had to tell people!