I will be SO let down if they don't have a skill ranking system that goes up and down with wins and losses, respectively. The 1-50 system is what kept me playing halo 3 for years, still play today. If you don't have a goal that takes skill, effort and consistency to achieve, you just have a ranking system like Halo Reach.
Playing Halo Reach, no matter what rank I achieved, I NEVER felt accomplished because thousands of people have already reached that rank, playing campaign or firefight. (not trying to take anything away campaign or firefight) but the ranks are achieved solely based on time played, NOTHING ELSE.
Reaching 50 on Halo 3 (as MANY gamers already know and would agree with wholeheartedly) gave you such a HUGE sense of accomplishment and reaching that 50, as stated earlier, requires skill, effort and consistency. We need this ranking system..