And nothing is preventing you, in any edition, from running lower level characters next to higher level characters, or adjusting the rate of XP gain. I'd argue that it can lead to a lot of in-group tension, especially if people aren't on board with that from the very conception, but that's edition neutral. If you're arguing that the level scaling in 4E was a tad intense, well... yeah. It was a problem in 3E too, but not nearly the way it was in 4E, and I think it should be changed. That being said, you can knock out the scaling 1/2 Level to both monsters and PCs fairly easily. Not saying that it even should be in there in the first place, but I don't see it as the sort of dealbreaker.
As for shoving more words in my mouth, please stop it. I don't want D&D to be "WoW on paper." I want it to be D&D. I'm actually a little disturbed that we apparently can't have this discussion without you assigning to me all sorts of opinions that I don't hold.
P.S. How many campaigns have you run with asymmetrical characters - characters of wildly different levels? How did they work?
P.P.S. Every edition caused an edition war. If you dredge up the old dragon magazine people were panicking that AD&D would take away "the D&D they love."