I don't really want to get into it, but I don't wholly agree here. I think it held for some cases. But swordmages played differently from wardens,
Oh, I don't doubt that they played differently, but that was certainly because of the powers. If you had given the swordmage the ability to nail people down with the Warden's grasping mark, and added some arcane fluff to it, it still would have felt right. If you had given the warden some tree striding or earth moving abilities in order to use the swordmage's mark, that would have felt right too.
All the defenders had a mark ability, all strikers did ranged or melee damage to single (or very few) targets, all controllers forced movement and took away actions and all leaders healed. So there could have been a lot more swapping of powers and builds between the classes and it still would have worked. It is not hard to imagine a fighter-esque warden who takes fighter powers, but keeps his "nail down everything that moves" central mechanic. A bit more difficult to see the other way of course, but that's the flavour text of power sources rather than of classes.
The powers also had things that interacted with the class to make them play differently that didn't really need to happen. Take the artful dodger's mobility. If the mobility powers hadn't been based on charisma, thuggish rogues or even assassins could dipped in to the rogues powers be a little more nimble, and the result still would have worked.
Going back to Defenders, I can take completely refluff the Swordmages' Aegis of Shielding as a form of barkskin, the warden's grasping mark*, the Paladin's Divine Challenge as Call Lightning, and the fighter's weapon challenge* just because I use a big club. Then you could take powers from each of these classes, and by in large it would still work if you were allowed to use the same ability score for all. It would also, with a little flavour refluffing, still look like a druidic fighter. The powers reinforced the build of that class, this is true, but it is easy to see that they could have stood on their own.
Now is this mixed class going to play exactly like each of the four classes? Nope. But the more powers you choose of one class or another, the more it is going to play like that class. Powers, to put it simply, are simply more important for mechanical differences than the class itself.
*(Apologies for getting the terms wrong, but hopefully I'm understandable, I've already sold my 4e books. I remember Paladins and Swordmages because I liked them better.)