In terms of comparison to other FPSs, my observation is that Halo, especially later on as other shooters gained popularity, didn't really do anything revolutionary or different than what others were doing. What made Halo stand out is that the things it did do, it did very well. Which, by all accounts, they're continuing with Reach.
Unfortunately, I never did finish all the games. I've beat Halo 1, and got up to the Flood portion of Halo 2 (I.... HATE.... Flood levels). I've got Halo 3, and I've played online with it, but never played the single-player campaign. Also never played ODST, and I'll probably wait until I finish the rest before I pick up Reach.
I do like the storyline, though, which is why I play them. Shooters aren't really my thing these days, but I do appreciate a good story.