I think the most pressing issue that 343 needs to fix is this P2P garbage. There are too many people who have sub-par internet connections for whatever reason (not economically feasible, can't access faster speeds, etc) and have actually dampened the experience for the faster users. Either dedicated servers or a way to divide games based on speeds would be something to strive for.
Personally it's frustrating for me and I'm sure there are others who feel the same. I guess I could just run Utorrent or another heavy bandwidth app to scale down to level with the others, but I pay a pretty penny for my netz and would feel wasteful if I did that! =)