4th Edition's biggest flaw was that they made every class the same. Sure, the words were different and the descriptors were different, and there were a small handful of semi-unique options available. But in the end, everyone had the same general array of abilities and actions available. They weren't really individual classes so much as different builds of a single class. The fact that they were all combat-oriented with little to no out-of-combat options didn't help matters either. Nor did the simplistic, overly redundant language they used in describing everything.
Say what you will about the quality and production values of earlier editions of the game, especially the first edition of AD&D, but at least TSR didn't dumb down the language or overly simplify everything. I remember being confused as Hell about some of the terminology used in those older books, and how they forced me to increase my vocabulary and reading comprehension skills. But the most recent edition? It makes me feel like I'm reading See Spot Run. If the creatively stifling rules and the MMO-oriented concepts weren't enough to drive me away, that would have done the job anyway. I don't enjoy being talked down to, and that's exactly the impression I get every time I read a 4th Edition rulebook... and the D&D Next playtest material.
That said, I think the biggest mistake they made and are continuing to make, at least from the playtest material I've read so far, is that they're catering to the
munchkins "optimizers" too much. There's a reason these games have living, breathing Dungeon Masters capable of independent thinking. WotC needs to tell the munchkins to sod off, obliterate their "character optimization" forums, and just focus on making the best game they can while trying to keep it as internally consistent as they can. To hell with the rules lawyering crybabies. They, like most everyone else, are just going to house rule everything away anyway. So why make a shoddy game that caters to the worst elements of the player base?
The fact that "D&D Next" is one of the most insipid titles ever doesn't help, too.