Ragnaros makes sense, its not really a retcon. When you beat him in Molten Core you're fighting a weak version of him that has been only partially summoned into Azeroth. When you "kill" him, you're just banishing him back to the plane of fire and basically screwed up all his plans for materializing in Azeroth proper. Now that Deathwing has broken the seals on the elemental planes Ragnaros is free to attack Azeroth directly without worrying bout his minions botching the summoning (I don't know how much you know about the MC encounter, but basically the boss before that is the guy in charge of his summoning and when you beat him he surrenders. He summons Rag early for you afterward when you ask him to).

Now Nefarian being back in Blackrock Descent… that I'm curious about. I distinctly remember looting his head and sticking it on a pike.