Describe? It's hard to imagine how being unable to go past PL for pretty much anything you have to roll for is not balancing.
The Limits of PL explains the problem in detail.
The problems I experienced were Autofire and Power Attack, combined with All-Out Attack, invisibility and Master Plan. Together these increase the 'to hit' for a PL 10 character by +13 (assuming +3 from plan and a defence 10 foe so halved defence and +2 to hit from invis), a huge value in a d20 system. Putting five of that 13 into power attack still lets the character hit a defense 10 opponent on a 2 or better. That means autofire will give +5 damage almost half the time, meaning the character was often dealing 20 damage instead of 10.
That same character, a speedster, was able to do slam attacks, which also ignore the PL cap, dealing +10 (his speed value), or +14 depending on one's interpretation of the rules, damage *on top* of all that. It's quite hard for the bad guys to make a toughness save of 49. Even if you don't allow autofire with the slam and interpret the rule as not including the +4, that's still 25 damage, or a toughness save of 40.
I banned slam attacks and eventually realised that I should ban this PC from using All-Out Attack, as he would use Move-By Action to hit his foes then run about 20 miles away, meaning he was unlikely to experience any drawback from A-A A, it was essentially a free +5 to hit for this character. The player wasn't happy with this at all, as we'd been playing for about ten sessions by this point and doing a lot of damage had, in his eyes, become an integral feature of his PC.
This is what I mean by having a tough job making the game work. Dealing with all this took a lot of my GMing time. Going thru the numbers to try to explain to the player, fairly unsuccessfully on my part, that this was a problem, considering solutions, etc.
Incidentally, builds intended to take advantage of Slam - flying cannonball types - with high movement powers, Move-By Action and lots of Immovable to negate slam's weakness, seem to be fairly common from what I've seen.
Another very cheap and effective damage booster is Improved Critical. Nine ranks of this, for a cost of nine points, will make every hit against a defence 10 opponent a critical, increasing damage by +5. This also ignores PL. One could even combine it with the tricks above, though that didn't happen in my game.
Dropping opponents from a great height, achieved with teleport in the game I ran, also ignores PL and has a maximum damage of 20.
This is why I think the PL system is worse than useless, because at first glance it seems to provide balance but once one has greater familiarity with the system it becomes apparent that it doesn't.
What it does do however is make certain types of PC - those that can justify Imp Crit, autofire, PA + AAA, slam builds, etc - *much* more effective than those that can't. In M&M, the Human Torch's nova flame doesn't do a lot of damage, because it doesn't get any of these PL breakers. Even the Hulk only gets Power Attack, making him far feebler than a slam build, and equal to a teleport-dropper. And that's assuming the Hulk goes for a +5 to hit, +15 damage split.
A better way to do it, imo, would be to look at the final damage values the system is outputting, rather than the initial, pre-PL breaker ones. Alternatively, just dispense with PL altogether and look at the final character, taking all abilities, including non-combat, into account and making a judgement call. This is hard to do though. It's very hard to give the players a sense of what power-level you are expecting this way. Also often a PC's power won't be immediately apparent, it can take a few sessions. And by that time the player may have got too attached to his PC as is.
