Well, Halo is basically to the bald space marine FPS genre, what C&C and Warcraft are to RTSs. Maybe nothing new, but it does it really well, particularly multiplayer. At least compared to other console FPSes.
PC Gamers probably wouldn't be overly impressed. It's sort of like Halo 2 was - it got released on the Xbox, got rave reviews. When when it came out on PC 2 years later earlier this year (using advanced Vista only technology, heh), everyone yawned and it got mediocre reviews. Part is the time difference, of course, but also is simply more and better competition on the PC.