How would you imagine it would be broken? Being able to do fantastic things is part and parcel of the genre, though it does require restraint on the part of the players to stick to genre.
While this is true, to a point, of any genre / system, I think this is exactly what causes problems in supers games. Personally I like PL, it's the tightest such system I've seen in a supers RPG, and it's specific enough that IME you don't get players trying to ignore it or work around it as in some other systems. But PL is not the same thing as Level in D&D.
Concept has to come first in M&M character creation, and PL is part of that concept. This isn't a concept that I found easy to come to grips with simply from reading the M&M rules, and I kind of wish they had done a book specifically on character creation for 2e. I found reading the ATT, and especially the Roll Call forum to be very enlightening. Specifically, seeing different builds of the same character at different PLs. PL8 Batman is a different character from PL10 Batman, or PL12 Batman.
The problem that I found I was having when I ran a bunch of 3.5 players through M&M character creation was that some of them where not really trying to make a supers / comic-book character. And even among those who "got it" no two players had the same idea of what the genre entailed. There's a lot of different sub-genres represented in comics, and they all have different expectations regarding the characters you'd find in them. Even a given character is often depicted at many different levels of power and possibly in different sub-genres within different books.
One thing about M&M is that GM approval is a much more important step in character creation than in most other games. Usually in D&D the DM can simply disallow elements he feels might be or might become (or even just are) a problem. Supers games are generally open-ended enough that this can become a much more significant issue. In M&M, for example, there are a lot of ways to push past PL limits, that's a deliberate part of the game's design. But a character who's hitting multiples of them, like the speedster Doug McCrae mentioned, is a problem waiting to happen. The optimal solution, I think, is to re-design the character.
I had one player who was really enamored of autofire, and really wanted to make a martial-artist with an autofire punch, a high attack bonus, and a very consistent ability to feint. I was willing to allow it, but only on an attack that was otherwise below the PL cap, and I wouldn't let him buy more than one "point" of autofire (ie: no 1/1 ratio or +10 max). He grumbled a bit, but I think the character turned out well.
The thing is - you don't need to do this sort of thing (or at the very least not much) in most non-supers games. If every player shows up with an X-level character, and no-one's used any rules sources I asked them not to, I can be pretty sure that everything's going to work out, most of the time. I find that supers-games as a rule are less robust that way. What I like PL for is, as I mentioned above, part of the character concept. It helps define which Batman (ore Spiderman or whomever) you're working with. And it also fills in some of the numbers for you, which given all of the possible choices in M&M is rather helpful in and of itself.
There was another player that I had an even more serious problem with. After a whole lot of discussion I came to the conclusion that he was only ever going to compare his character to the other PCs, and as a result would never feel that his character was a "real superhero" unless it was just flat out a higher PL than the other characters. I ended up having to un-invite him to the game because, despite his protests otherwise (which all included the caveat that I just let him play the character he wanted), I didn't feel that I could make the game fun for him and the rest of the group at the same time. (I know you can totally run a game with different-PL characters, but there where two other players who flat-out didn't want to give that a try and I think the character he wanted would have been too much above the other PCs for it to work well.)
Anyway, to get back on the topic of 3e: it really helped me to have the builds on the Atomic Think Tank forums to look through, seeing how different things could be done with characters I knew something about. Seeing Batman or Superman statted up is more useful to me than looking at the Costumed Adventurer or Paragon archetypes. So I'm hopeful that character creation in 3e will be that much easier to show / teach with the DC roster books.
But I'd still love to see a full-on "secrets of successful M&M character creation" book. I would also, especially given that M&M is usually a team game while most comic-book superteams are made up of characters that have or at least had their own books, like to see something more in regards to making a functional team as opposed to a bunch of loners. Something like the Roles in 4e D&D would interest me very much, although I freely admit that I have little to no idea how to bring that concept to M&M...
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