It's well known that Halo 3 have some issues in balance, not only between weapons but also other stuff wich I will come to later on. But first of all.
The weapons (covenant/human):
It has been widly discussed if the BR should be in Reach or not and I'm not a huge fan of the BR but I could live with it if it would be less OP. And in my opinion, by deleting the BR would not make the balance between weapons much better.
The thing is, that human weapons are pretty much "aim & shoot", you don't need to consider any bullet velocity or illumination curves, just aim at the target and shoot. And I find this slightly frustrating because it's pretty much the one who gets the first shot who wins.
But this could be solved by adding more recoil and making the gun "stiffer" to aim with while shooting, this way more skill is needed to get a kill with a human weapon, unlike how it is today with the AR and BR.
And for the Covenant slow-moving projectile weapons-> increased bullet damage so in a Head to Head fight (Covie/Human) the Covenant would win.
The next topic is about the Vehicles.
I don't have as much to whine about regarding the vehicles as the weapons. But there are a few points wich I would like to see improved in Reach, and thats the Plasma Cannons' bolts velocity, damage and spread. Lets take the Warthog and Prowler for example, the Prowler has a much weaker turret than the Warthog and it is also (of course) much harder to hit your target at a distance with a plasma cannon. So my wish here is to make the turrets equally powerfull and make the plasma cannons more like the Spectres in halo 2.
Same thing with the tanks. On a flat surface, in a Scorpion vs Wraith battle the usual winner is the Scorpion, due to it's accuracy and shell damage. A solution for this case is to make the reload time longer for the Scorpion and maybe the Wraith's plasma fire stronger against vehicles.
Matchmaking:
A rather annoying thing in matchmaking are the Weapons on Map. The majority of weapons are usually the same weapons you start with (BR AR). And a lot of weapons are seldom eaven placed on the map, like Carbine, Plamsa Rifle, Beam rifle etc... And I do not see the point in putting the same weapons on map as you start with :P So what I would like to see is moar COVIE WEAPONS! No just kidding (almost), but yes. More variation and a equal amount of weapons on map (C/H).
And one last thing
I think all of you have noticed that Spartans has much more body armor options, wich I do not really like because I prefer playing as an Elite.
And that was all I had to say. I hope that you haven't found this too depressive and managed to read it through.