Bioshock looks like it will be a fantastic RPG. It reminds me of both the early fallout games with their off-center technology and setting as well as the ominous mystery of System Shock.
Technically the developers describe the game as a first person shooter. They have put a great deal of effort into refining the combat. Character development seems similar to Deus Ex. Rather than gain experience and level up you will be able to obtain various enhancements to your character as you play through the game. Through genetic altering tonics the player will be able to acquire special powers, gain strength and health, learn to manipulate machines, and become better at both dealing and avoiding damage.
The interface has a very steampunk feel to it. Much like fallout did the Bioshock UI combines simple seeming images with intricate but worn-down mechanical elements in an eerily familiar way. There are no flashy LED readouts or modern backlit displays. Instead the appearance is more like what you would find in a 50's vision of the future with bakelite parts, large knobbed dials and old fashioned incandescent lights.
The story of Bioshock has the potential to disturb as well as entertain. The setting places the player as an outsider in an underwater arcology called Rapture. Long ago something happened that transformed Rapture from a thriving, exclusive community of scientists and artists into something you would expect to find between the twilight zone and a horror show. The people there have been genetically altered beyond the point of being human and established a social order that is unlike any other.
It is up to you to find a way to survive in this alien environment. You will have to learn what happened and decide how to handle it while you search for a way to get back to the real world and the comfort of it's familiar logic and morality.
There is an excellent flash site that will familiarize you with many of the games elements.