We knew of Rags in vanilla. The zones, the Dark Iron and of course the MC raid all established that lore. So when he appeared in Cata it wasn't "Here' a new enemy! Ragnaros!!!"
That's what I mean. Obviously they can and need to create lore of things that happened post WC3 - but for it to really work they need to lay some groundwork. Hell, they even did this in a minor fashion with Garrosh. So when people say "they can just pull something out of thin air! It's no problem creating new lore!!!" I disagree that they can or should just spring new threats with no background at least not if they want it to be well done.