M&M PL balance: Picking up the bazooka
If you play M&M at “agent” level (lower than the usual PL 10), you are more likely to run into cases where players pick up equipment during play that breaks PL.
Commander Orion is PL 5, so he's built with +5 attack and a +5 damage Pulse Pistol. During play he knocks out a space pirate and nabs his Zeta-Ray carbine (+7 damage), so now he's effectively PL 6 offense.
As the GM it's an awkward situation. You don't want to encourage players to just fish for bigger and bigger weapons (particularly if doing so goes against their character concept), and it may also feel unfair to other players who didn't get the big gun: PL balance is just as much about balance between the players as it is between the players and the things you are having them fight. You can just forbid the players to pick up new weapons, but it's hard to explain in-game why the heroes don't pick up the obviously bigger guns, and it will probably feel arbitrary to the players.
A simple solution solution is to let any character pick up any weapon that makes sense, but reduce their attack so that PL balance is still maintained: it's “too much gun” for a lower PL character, so they lose accuracy trying to use it.
Commander Orion is normally +5 attack / +5 damage. When he picks up a +7 damage rifle his attack drops to +3, keeping him at PL 5. He's just not used to the heavier weapon.
Even with this penalty, there are cases where picking up a big gun can be quite useful: you may hit less often but you do more damage when you hit, and a heavier weapon might be a better choice against a tough target. Acquired weapons might have other bonuses that are not directly related to PL, such as better range modifiers, autofire capability, etc.
The same thing can apply to other PL issues like defense / toughness: the character finds a suit of heavy armor and puts it on, but suffers a corresponding loss to defense.
You can let characters exceed PL with some weapons if there is a corresponding downside that sets them apart from normal personal weapons like pistols or rifles. A heavy machine gun that requires a tripod placement to operate and needs a few rounds to set up is fairly limited, so it's fair that a person using it is more dangerous than if they just had a rifle.
A heavy weapon can exceed PL limits by +2 total (just as though the user was one PL higher), and even more unwieldy weapons like light artillery could exceed PL by +4 (two PL higher). The stats of the weapon itself don't change, this bonus just determines how to reduce the PL attack penalty for characters using the weapon.
A gang of PL 4 space pirates have boarded the Antares. They normally carry +5 damage Zeta-Ray pistols and have a +3 attack. A few carry carbines (+7 damage) which automatically drops their attack to +1.
The pirates break into the armory and get their hands on a tripod mounted ARC beamer (+9 damage autofire), which they promptly set up in the hallway to control access to engineering. Normally the PL 4 pirates would be at -1 attack with a +9 damage weapon, but because it is a heavy weapon and can't be easily carried around, the GM decides it can exceed PL by +2. The pirate firing the beamer is therefore +1 attack / +9 damage, just as though he was PL 5.
Characters should still obey PL with any equipment they are “built” with, or you could do the opposite and permit them to exceed PL at the start if they are willing to lug heavy weapons around.
This rule emphasizes PL as an overarching measure of a character's competence in combat, which I think is appropriate. If you want things like feats for weapons familiarity etc, you can do that and just apply additional penalties when the character is not proficient, but buying such feats should not let you exceed PL for all the obvious reasons.
There is the odd case of a high PL non-combat character winding up with a better attack with a lower PL commando/ninja type: the PL 8 occult librarian might start with +2 attack, but it won't go down even when she's using a +8 damage bazooka, but the PL 4 commando will be at +0 attack / +8 damage. Just consider it a perk of being high PL.
And no, for all you Rocket Ranger fans, Commander Orion is not really PL 5.
update: These rules have been published in Green Ronin's Agents of Freedom supplement.