I also agree there should be the Halo 2 clan organization, for playing with friends and having good people to play with, not just random people who are horrible/rude.
The Halo 3 ranking system, because players actully have an incentive to win, and will actually play against players who arn't much better or worse than you, plus it's always nice to have a high rank in playlists.
And finally, the Halo Reach credits system buying things in the armory plus whatever Halo 4 has to buy, but you should earn more credits for winning of course.