I kind of miss dual wielding SMGs.

 kart racing 

Take off limits to how many players can join certain maps in matchmaking.   Its annoying when after 5-6 friends log on we have to switch to BTB and there's only a few maps and modes to play.  The larger maps are way too big and boring, and it'd be cool to play "the cage" with a ton of people, even if it'd be really crowded.  I don't know why their restricted, in COD sometimes you get rust in the rotation even though there's probably too many people for it.  I also don't like the single weapon modes in matchmaking like Snipers or Rockets only.  I don't understand why people would want to play those competitively, they feel like something you'd do with some friends in custom matches.  Lastly add classic slayer to Arenas.  When I think of Halo 1,2,3 I imagine starting with a assault rifle and a pistol.  Its super irritating to only have 2 modes in arenas that involve Battle rifles.
 
On paper Halo's matchmaking has TONs of options and it seems great, but when you actually play for hours on end it just feels like your playing Snipers on Asylum all day.
 
imo  :)

Make co-op work on 4GB Slims.

PS3 release. Not saying it would be better on PS3 - just I'd actually be able to play the full thing without being restricted from the online portions on my 360.

Make better playlists.
 
If I pick Team Slayer I don't want to play Headhunter/SWAT.
If I pick Big Team Slayer I don't want to play Big Team Headhunter/SWAT. 
 
Can't we just have a slayer list like in Halo 3? Simple 6 on 6 slayers or ctf's, anything but Call of Duty mode.

I've only noticed lag in the first cut scene so far, so personally I don't think that's an issue.  At the moment I can't think of anything, but I'm sure there's some nitpicky stuff I'd discover later on that I would.

make it easier

Simple. Let us decide what we want to play online. Whether it's a PC-esque server list, checkboxes in a menu (I'm thinking Smash Bros. Level/Item customizer,) or some sort of custom game creator/search with matchmaking, I would absolutely love to see some option of control over what games I'll be ushered into. The voting system is a nice touch over their previous attempts, but I'm still being forced into really stupid gametypes against my will on a regular basis.

Don't get me wrong, I can get behind a good game of Snipers when the mood is right - but when one is in the mood to play Halo (spawn, crazy gunfight, die, etc...) it kind of sucks to get put into what boils down into a very quiet game of Hide-and-Seek for 10 minutes.

Besides that, I think the game is pretty phenomenal. 

Two things.  Minor Spoilers, but if you looked at any of the screen shots you know the covenant are in this game.
 
First, I would alter the first mission to actually be against human separatists.  I was so excited when it looked as though the first mission might be against rebels... I really wanted a situation like the covenant invading in the midst of a pitched firefight with rebel humans... perhaps forcing an uneasy alliance between the forces that could have added more depth to the meta story, or made the Spartans look that much cooler for winning.
 
Second, rebalance the shotgun around a faster rate of fire.  It's the only gun in the game that really just doesn't feel right to me.

Dragons.  Dragons make everything better.

-Getting rid of the quitting ban
-Make friendly AI actually useful
-Make weapon damage actually consistent 
-Making SWAT its own playlist

Making more modes their own playlist, that's it's only real downfall for me.  I can never get into the games I want :(

" Getting a solid framerate in cutscenes would be a nice start.  It could also use a bit more build-up to the introduction of the Covenant, and more time spent with Noble Team getting to know them before things go sour. "

Change the driving controls.  The driving in this game makes me want to yank my hair out.  I wouldn't mind if they assigned the acceleration and brakes to the triggers like driving games.  As it is, I find it unresponsive and restrictive having the steering and camera control on the same analog stick.  Don't know if I'm the only one who feels this way, as I haven't seen any complaints out there on this.  Other than that I am thoroughly enjoying everything else. 

Any non gimmick gametype (like rockets/snipers etc) is broken if you don't start with at least a needle rifle or dmr.  The AR is as useless as halo 2 smgs.  Figured they would have had learned their lesson.
 
Get rid of swat, if you want twitch go play one of the other 50 shooters that are about twitch.