Id say Halo 3 was important, It stressed the importance of community. Forge is not only a real time map editor, its also a multiplayer real time map editor you can do with 7 other people at once. File shareing is also huge, you can trade pics, and custom maps. It also had 4 player online co-op. Halo 3 is as important as any of the other Halo's.
None of that is revolutionary lol, I hear forge tossed around alot as the saviour of innovation, it's actually pretty lame in my opinion. The game still has nothing that separates itself, I found Tribes, a game I played in highschool to offer me a much more personalized experience that halo ever did. Jetpacks + skiing FTW
From 0 to KICKASS in .stupid seconds.