Let me just throw my two cents out there if Bungie is listening after my experience with the beta, and please take these seriously:
I would replace (on oddball games) "ball dropped" with something else; it's somewhat suggestive.
I would dumb down the frag grenades; they are WAY overpowered.
I would remove sky limits for jetpacks, because you lose your forward momentum whenever you hit the ceiling.
Carrying the flag makes you much too slow; I'd speed up the flag carrying a bit, as well as the oddball.
I'd make the score accessible before the game starts and between rounds, when there's the box that says "score 150 points" or "50 kills" because you can then mute others before the gameplay begins without wasting time.
I personally would get rid of the whole moving while camo'd makes you visible thing. It's good that you become visible while you use a weapon, but moving makes the whole thing worthless. This way it could be an offensive as well as defensive power. You could even make it last shorter.
I would improve the sticky targeting. When it comes to using the precision rifle as well as the needle rifle, it's much to challenging to "four shot" someone like you could with a BR.
I would remove the one melee to the back to kill them assassinations, and instead entirely rely on the assassination animations. If you have both, the animation is worthless.
I would change the HUD for the grenades. As is, I noticed I never felt the desire to pull out the stickies for the purpose of sticking people but rather just to use to clear an area. I'd make the grenade HUD more like in Halo 2 and 3.
I know it's a big new feature for Reach, but I much preferred the system in H3 of viewing service records over the new way of everyone being put in a list together when you press right while in the matchmaking room. If I want to see what someone looks like, I want to see it individually, not while everyone is together. Besides, you can't view the voting poll when you're doing that.
As Ryuk mentioned, I would either make sprint longer or make it available for all classes.
I would make a setting in the start menu that allows you to choose a favorite ability so that whenever the game starts, the cursor is always selecting that ability. "Preferred Ability" perhaps?
This is just a little addition, not really a suggestion...but I really really don't like health packs. It was a bad idea in Halo CE it's a bad idea now. You had it right in H3.
I would change the medal for "Opportunist" because that smiley face isn't cute. The killjoy medal is cute. I'd change the opportunist picture to actually have to do with the medal.
In the post game stats, I'd get rid of the medal list altogether and replace it with what you had in H3. I'd like to know how many of each medal I got.
I'd make the hills (KotH) taller so that you could be flying just above the ground and still be in them, but not at risk of getting assassinated by a camo.
That's all I've got. If any other posters agree with me, please say so in a response. Overall, great work Bungie!