Being one of my Top 5 PC games of all times, I really have to advertise this whole deal. System Shock 2 is a bit like Bioshock, but with much more of a PC slant. This means stats you must level up, an inventory system, and intense exploration of each level. Not only a shooter, but more of a space survival horror game, you must fight your way through mutated crew members, rogue robots, and even greater horrors.
If this sounds complicated, don't worry. The entire game is mouselook controlled with a UI overlay that can be removed or retrieved at the touch of a button. All of the stats are under the hood and mostly serve as barriers to using more advanced weapons. All you need to worry about is surviving. I have mentioned this game repeatedly as a touchstone for good pacing, appropriate use of horror themes, and balanced gameplay. If you enjoy PC games at all, I sincerely urge you to complete this one.