Well, let's talk about what was good about 4e's approach to roles.
Some roles have always been implicit. Take defenders, for instance. It was no major revelation to tell folks that fighters and paladins should be on the front lines protecting the softer characters. I think it's to 4e's credit that it made the job of defending the party consist of more than just being the closest target to attack.
They did another good thing in providing a healer role (ahem, I mean "leader") that led to all sorts of interesting ways to provide support to the party beyond spamming cures. I don't think bards have ever been cooler than in 4e.
And best of all, thinking along the lines of roles helped give purpose to some classes who previously lacking it. Monks and rangers, for instance, never had broad areas where they excelled. Monks had a lot of dumb, contrived abilities and rangers were just fighters that could track.
Conversely, classes like cleric and druid that encompassed virtually all roles were reined in. They could no longer do battle on the front lines better than a fighter while throwing out artillery spells.
So, where did they start to go downhill? I'd say it was with wizards and the institution of a "controller" role. While there were always spells that were good for crowd control, D&D did not previously have a class purely dedicated to it. I, for one, don't think it really benefited from having one foisted upon it. Combat is just not that fun when it constantly consists of fighting a bunch of dazed, immobilized, blinded, punching bags.
The appeal of a wizard is not that they chain-stun or drop AoE's. The true appeal is that they are a swiss army knife of utilities. They are a like a deck of Magic cards that, with proper preparation, can bring the party through any situation.
Rather than a controller role, the designers should have thought along the lines of a role that was designed for versatility and problem-solving. But that fell outside of their imperative to make classes uncomplicated and homogeneous. That was a major mistake. There are always going to be players who want something easy to play, and there are always going to be players that want something intricate to play. Somebody needed to stand up and say "y'know, it's okay for SOME classes to be more advanced than others".