Hmmm, let's see...
-Armor Lock(Killed Elite Slayer on
any map even BTB, and also killed pretty much any 4v4 slayer on smaller maps)
-Reticule Bloom(There needs to be an option to remove this in Customs)
-Shitty forge map remakes(I could build a better Sactuary remake with Legos

)
-No Plasma Rifle or Spiker in matchmaking(Why the fuck did you even put them in the game Bungie? This makes NO FUCKING SENSE...at all)
-Customs is dead(At least on my friend's list and several other people I've talked to)
-AFK/Party Boosting in MM(Go back to griffball you fuckers

)
-Arena ranking system is crap
-Noble Map Pack was a waste of money.
-Weapon sandbox is dead

(Although it's been going down hill since H2 it's not like it suddenly dropped dead.) In Halo:CE literally
every weapon (with the exception of the needler) had a niche to fill. This would've/shoud've been the case with H2 as well but RRXYYRRX shots and BXRs pretty much made the BR a god. H3 actually had a decent weapon sandbox IMO but I didn't care for the game as a whole.
-There are also several things in the Campaign and Forge that break Halo canon and continuity. Of course these things don't affect online gaming experience but they sure as hell bug a nerd like me

-Oh, and I seem to remember seeing a video on b.net prior to launch(you know when all that beta hype was fresh)that bungie had made talking about the cool stuff they've added to Reach. And one of the things that stuck out to me and I though was really cool was MM Chess Halo-style, but as you know this has yet to be seen. Very sad I love Halo and I love chess.

I'll look for the link later it should still be in bungies archives.
...definitly more gripes about this game, they're just not mine

(hell, just spend 10 minutes or so browsing bungie.net forums and you'll see a million more)
If pushed though I would say armor lock is the most pressing issue here. This needs nerfing/fixing ASAP.
Don't get we wrong, I've played a shitload of this game already(General Grade 1) and I have lots of fun on BTB and Invasion. And there are plenty of things to praise about this game. I absolutely love the new forge.
Expect some cool updates next month though(for one thing bungie is fixing The Cage and the power weapon spawns/ammo on Atom) and bungie did say they will continue to support Reach with patches/updates for
at least as long as they did with Halo 3. So we'll see what the future holds...
For the tl;dr this sums up everything about Halo :
H1>H2>HR>H3
Or a shorter, and IMO more accurate version :
Halo 1 > Everything > Halo 3
edit - If anyone is interested PM me and we'll exchange gamertags.
@Atri - Have you actually installed the game onto your harddrive. This should significantly decrease load times and some other lag/latency issues. I don't know if this will help or not I don't know much about computers. You say you can't get into MM/Campaign/Theater, can you use the forge? play customs? play firefight? Cause if so then it's probably your disk not your harddrive. But again, I'm a computer novice so I may be way off....actually no, what comes below novice? Cause that's what I am with computers :D