Somebody said it before, but it creates the most powerful atmosphere in pretty much any game ever and is a master of diabolical mental manipulation. The big 'reveal' is probably the most memorable dialog in any game released thus far. The gameplay, which was pretty creative, didn't need to be revolutionary in the face of that, just executed well, which is exactly what it was. I'm not a big fan of FPS in general, but this game is beyond an FPS.