@Arkandel In my experience, you could run a scene fairly comfortably for characters that are within 2 PLs of each other. But that's really a very vast generalization. Power Level is really a murky barometer at best to compare characters by. Sure, on paper, 2 PL 10 characters should be pretty even. Except this PL 10 character is only PL 10 because they've maxed out a number of skills and have a PL 10 mental power. Their defense comes in at PL 6 so when they get punched by this PL 10 brick they go splat. Power Level is something that needs to be carefully balanced and weighed by everyone involved in a game/scene. You can't really make a generic "This is a PL X situation". More often than not I'd have to promote scenes like, "Ok, so in this scene there will be PL 10 physical attacks being thrown around all over the place and up to PL 8 mental attacks, and PL 9 puzzles to solve." So people could decide how comfortable they were with their character entering a scene. And even then the game unbalances fairly easily when certain combinations of feats and powers are used.
As an aside, this is why my group switched to Supers RED. They have a built-in mechanic for balancing characters of different levels that we found works way better. Characters are rated on the number of dice they're built out of. So say we have a scene where this guy wants to bring his favorite power level 20 dice character into a scene with a bunch of 25 dice characters. Instead of just doing it and "sucking it up" he's given a pool of extra dice to help compensate for the power difference to show how he's taken measures or beefed up to play with the more powerful heroes for a while.
And, again, these are all just things from my own experiences playing and running M&M both online and in the real world.