And don't get me wrong, I'm not dumping on M&M. I really enjoy M&M. My TT group uses it for our DC comics campaign and used it regularly until we found Supers RED. I'm not even saying that it shouldn't or can't be used to run a game. I'm just sharing my experiences from running a game using M&M in the past.
1: Some people won't play there for various reasons because you're using a system/because you're using M&M.
2: I would set 1 PL and have all characters make characters at that PL. Or have very defined guidelines for what types of characters fit into what PL.
3: Be prepared to have to really help people understand that they should have both offensive and defensive stats that equal X. Here you start getting into equations and some people are going to tune right out. On my game, I had a command that broke these things down as best as we could.
4: If you're using 3E be prepared for people who do not like how Impervious works. Also, some people really dislike the utility power Affliction. There will be debates about it.
5: Be ready to fully enforce a PL offset rule. Otherwise, certain players will sink one side of a PL equation to completely make the other side OP. Tanking Dodge to create an OP Toughness for instance.
6: Even if you try to account for everything(and that's really just not possible) the system can still be broken by people who are familiar enough with it. The idea of Power Levels is good in theory but I've found that, in practice, it's not as solid a mechanic as I'd like it to be.