You interact into this dying world where genetic modification has gone awry. You can select modifications called "plasmids" for your advantage. You have various levels that have goals, but there is no one true way to achieve these goals. The elements are not static, meaning that the elements exist for you to interact, rather than them telling the story and forcing you to come along. There are many ways to solve the same problem, and there is no one plot line you have to follow to get to any particular goal.
The whole game has this amazing art deco architecture which is very realistic. The characters are haunting, terrifying, and many of the elements are very disturbing in their realism and nightmarish quality.
The trailer from E3, Might be disturbing if the thought of drills going through you or bees erupting from your flesh seem like too much to handle (scares the shit out of me).
Here's a description of some of the gaming elements, like "Little Sisters" and "Big Daddies," plus how some of the elements can be creatively and flexibly used to your advantage. Slightly less disturbing, but more descriptive.
This demo shows some of the "plasmids" in action. In this case, you can see how some of the elements interact with one another. This one is slightly gorier than the others.
I have no idea if I want this game, or it is too scary for me.