While I have yet to play Halo 3, I've always found Halo and Halo 2 (which I did play a fairly extensive amount of on the original XBOX) to be generally over-rated.

I'm not saying the games "sucked" ... in fact, they're very compitent, and technically sound titles deserving of well above average critical scoring.

I just don't quite understand the "mass appeal" that they seem to have garnered (especially amongst non-gamers and non-FPS enthusiasts) over the years.

As far as FPS games go, they didn't to THAT much to advance the genre except bring that elusive sense of "cultural awareness" and "synonimity" (the same way people say "kleenex" instead of tissue, and "xerox" instead of photocopy) that no other FPS before it ... except DOOM maybe, has been able to.