If you have played the beta you realized that there are two systems of ranking in Halo 3. One is the levels that you obtain from winning in specific playlists. another is a different system that I like to call the Rep system. You are given a specific Rank based on how many Rep Points you have.
The system is pretty simple, and also just doesn't show you how many points you have it show you how many games you have played. I personally like the new system. Now what do you other beta players think of it?