In my opinion, the only time a weapon is overpowered is when it has to be unlocked to use it. If a higher rank gets a better weapon, that automatically makes it harder for people who've just started to reach that rank. Thankfully, this isn't quite the case in Halo (except maybe with the Light Rifle). All power weapons spawn in-game so that everyone can get them, rather than being a part of a players loadout. In previous Halo games, there was the argument that one team spawned closer to the power weapons than the other, so this gave that team an advantage (on asymmetrical maps), however in Halo 4 they've made the power weapon spawns random so now it's luck of the draw.
Weapons can only be overpowered if they make the wielder invincible for the entire game, in my opinion. That would be something like a long range shotgun that has a massive amount of ammo and an aoe insta-kill on contact. In Halo, all weapons that people have called "OP" have had at least one drawback that meant in the hands of someone who had no experience and wasn't any sort of skilled, it would be pretty much useless.
Here's a short list of some weapons that have been considered OP by the Q.Qry baby players over the years that have resulted in some very annoying nerfs, as well as why they were considered OP and the drawbacks that made sure they weren't actually OP.
1. The Halo CE Magnum: Three shot kill weapon, if you got the headshot all three times. Only has a 2x zoom, and a small clip.
2. The Halo 2 Rocket Launcher: Could lock onto any vehicle. Only has 2x zoom and can only lock on within range. Only has 2 shots per clip. Can kill it's wielder if not used correctly. Slow moving projectiles make it easy to dodge/bail out of a locked on vehicle.
3. The Halo 2-3 Sniper Rifle: One shot/two shot kill, can be fired rapidly. Very small reticle makes aiming more difficult than other precision weapons, and is only good in CQC if the player is skilled with it. Can kill it's operator with ricochet shots, and only has 4 shots per clip.
4. The Energy Sword: One melee kill. Has a moderate amount of ammo, but can be effectively countered using a shotgun. Makes the user vulnerable to team-shooting. Can also kill the user if they lunge and a ledge clips their legs.
5. The H2 Needler: Dual wielding, projectile tracking, and the super combine. Needler ammo is difficult to come by on some maps, and the projectile tracking is only about as much as an overcharged plasma pistol. Is also useless in long range combat. Can ricochet and kill it's user.
Here are the nerfs for the aforementioned weapons.
1. Damage extremely reduced, 2x zoom taken out, then put back in.
2. Ground vehicle lock-on removed, rocket drop added, projectile slowed, AOE reduced.
3. Fire rate reduced (by a lot).
4. Sword block added in, made rarer to find, armor lock makes this weapon very ineffective as well.
5. Dual wielding removed, damage reduced (amount of needles required for a super-combine kill increased), damage increased, range reduced, ammo reduced, difficulty in acquiring ammo increased, projectile tracking reduced.
Fact of the matter is that weapons are meant to kill people. When they are nerfed, they slow down the pace of the game.