The recent D&D podcast mentioned how they really put a lot of time into the fluff of Primal Power. It always surprised me by how little fluff is put into the divine power and martial power. But I always liked fluff...
About six months ago, I had a lunch with Rodney Thompson. I asked him about the Elemental power source, and basically "If so many classes all ready do lots of elemental damage (Wizard, Sorcerer), how is the Elemental power source going to stand out?"
His response was basically "We haven't even started designing the Elemental classes. But when we do, we're going to come up with a Story for them. That story will tie them to the world, and will reflect their mechanics."
You can see what he's talking about with the Primal classes. I expect the Psionic classes to be similar (Well, except for the Monk). The Psionic classes, it looks like, are themed around an awakening in response to the Far Realms encroachment.
I get the impression from the PHB that the devs were so busy trying to hammer out the rules and get everything set up, and also market it, that they didn't really
think about real unification. They wanted to get the classes to work, and get them out the door. I think that now they have "room", breathing wise, and mechanics wise, to explore. The framework that they made is rather strong, and now they can tinker with the framework.