I know there are of course people out there that are concerned with balance, but I would bet my life that it isn't as widespread as it's being made out to be.
This is of course a figure of speech, but I think you would just have lost your life right there.
I, like everybody else and as Umbran wrote upthread, cannot in any way put numbers on how many people think combat balance through the classes is important. All I can tell you is that combat balance was, in my play group, a VERY big "two thumbs up". I'm thinking especially of one guy who has played lots of RPGs but has never really read the rulebooks. He likes to choose strange choices because he feels that's how he can be more unique, the problem being that he tends to picks classes that are not as well supported or not as well known as the mainstays like the basic classes.
Just to give an example, he made a character with the goal of becoming a Horizon Walker in a 3.x campaign. Well, I'm sorry to inform you that after playing that character for about 15 levels, he was disgusted by how ineffective he was in combat compared to other PCs.
Because of combat balance, that never came up in a 4th ed game.
Of course, my experience is as anecdotal as anyone else's, but it goes to show that calling this a "myth" is simply an error.
XunValdorl_of_Kilsek said:
Balance does not automatically equal fun, 4th edition is a testament to that.
Singling out one element out of 4th edition and saying that edition's failure is due to it is a fallacious argument. Otherwise, you could single out any other element unique to 4th and attribute its failure to it.
Now, if your argument is "Combat balance shouldn't be the be-all, end-all objective of D&D Next", then I am right there with you. But if you're saying that combat balance would doom Next because it has doomed 4th ed before, well count me out.
Combat balance IS important IMO, but it's not the only important thing.
And at the risk of being controversial, I'll even go as far as saying that combat balance is MORE important in D&D than in a lot of other RPGs because combat is such an important part of the system, no matter what edition you're playing.