A UI overhaul would be my top pick. Right now, I think it looks nice, but it really doesn't make the grade for usability. Here's some of the main gripes I have with it:
It's missing quite a few functions that would be expected, both in preferences and gameplay. No sort of gamma settings (BTW, are you guys aware that H3 and H4 have incorrect gamma curves? Much brighter than the originals, might want to fix considering the X enhancements are coming.), audio settings (especially if we could get individual volume sliders for voice, music and sfx) and a bunch of different game settings. No team changing in the universal menu means that H2 Classic zombies custom game no longer works, as that requires players to switch teams.
Another issue I find frequently is the backgrounds and music. It can be very inconsistent and annoying when both of these change all the time -- as I'm sure you're aware, in the main menus, during voting or game selection, the background videos and music change based on the game. It's actually a really neat idea, but if you're moving through a bunch of different menus, the constant stopping and changing of backgrounds and audio is a little jarring. I think the ideal solution for this would be allowing users to select their background audio and video -- CE, H2, H3, H4, that it stays locked to, while also having a "Dynamic" setting that allows the user to have backgrounds function like they do currently, changing depending on what game is currently selected.
Not to mention the glitches. I've had some very strange ones that keep popping up regularly. Some times, I go to custom games and it deletes all of my forged custom games and gametypes, permanently (that can, luckily, be redownloaded from the file share). Some times, after pressing start at the first screen, the game does nothing for upwards of 30 seconds. And another strange issue happens when selecting a map for the games: the map images take a really long time to load (again, upwards of 30 seconds) and I can't select anything in that time. I'm pretty sure I still have a ton of videos of these happening, if anyone wants to see them.
But most importantly, the social features in menus are much downgraded from past titles. In Halo Reach, you could move the joystick to quickly move from voting to viewing player cards, quickly having a glance at all of their stats, and then expand them to see a detailed service record. It was extremely elegant and you could get a lot of information very quickly. But in MCC, you have to press a button to go to the menu, click on a user, select the option to view their profile, wait for the screen to load, and press another button to view their stats. It makes the social aspects a lot harder to use. It's also missing some cool features that we saw in H3 and Reach, where you could see a player's Spartan -- how cool would it be if the player could select a spartan from their favourite game to render on their profile page?
The bottom line is that the UI could be much more efficient, and less jarring in certain instances. I feel that having the universal MCC UI more refined will cause a lot of the negative feeling around the game to go away.