Alright, prepare for a REALLY long post.
The reason the Halo 3 ranking system didn't work was because it was based on w/l ratio, which meant that either you had to carry like crazy or be a team player to get a 50. The reason this ranking system was fundamentally flawed is because it penalized losing with a lost rank. You could go from 1 to 50 and back down to five in a week. The penalizing also lead to "deranking" and "boosting", which turned the entire ranking system into a lame joke. A deranker is someone who loses games to go down in the ranks just to troll the lower level players by not allowing them to rank up. A booster is someone who loses many many games intentionally so that whoever they play with will get boosted up 10-20 ranks at a time, depending on how good the booster is. For a while there was even a black market for boosters. This sort of thing is unacceptable (and also illegal by a couple of copyright laws), which is why this ranking system is no longer used.
As far as Halo Reach's ranking system, it too was flawed because it followed the "time played" route. Rather than rewarding ranks to exceptional players, it implemented a credit system that allowed players of mediocre skill to rise in rank. This ranking system wasn't just for Matchmaking either, campaign and other non-multiplayer gametypes also rewarded credits. This is why there are inheritors who are -30000 in matchmaking.
So these two ranking systems are obviously not the best choice for Halo 4. Unless, they were combined and changed a bit.
Here's my idea for this:
Ranking system based on levels 1-50 that is given base on individual player skill (k/d, medals, and whatnot) and a symbolized ranking system that shows time spent playing the game. This way, you know that you are in trouble if you see a lv50 Inheritor on the other team. However, the ranking system should only go negative based off of an individual player, to prevent booster accounts. There will still be derankers, but they won't affect things that much. They also shouldn't make it a one step forward two steps back type of thing for the 1-50 ranking system either, which will help against derankers. Also, armor customizations and such should be unlocked based on the symbolized rank, and symbolized rank should also be based off of skill to some level. The better you are, the more you rank up in both rank types.
This will appease both parties, the MLG and the Casual players, because they can both rank up, but the MLG players can still brag about how good they are without having to look up k/d ratios.