While a lot people agree with me when I say that the Reach ranking system was faulty, I think there are still certain traits they should keep for Halo 4.
1. The high amount of ranks, to reach the highest rank in Reach; you need to have played a ******* ton. I liked this, it helped differentiate between scrubs and veterans (not to be confused with skill). Unlike Halo 3, where it took an considerably shorter time to reach max-rank.
2. High amount of combinations of armor; this allowed your spartans to be unique while giving players a sense of goal, which helped motivate people to play more.
The system should be more focused on skill-ranking, but I'm sure they will figure something out.