As you know, the Cataclysm is going to bring major changes to the whole world of Azeroth. There will only be 5 new zones for leveling above 80 and one new zone for each new race -- the rest of the work they're doing involves changing the old zones, bringing them up to the standards of zones in The Burning Crusade and Wrath of the Lich King, adding new quests that are more appropriate to the current timeline, and completely rebuilding the areas that just didn't work so well.
You are also probably aware that this is a much-needed improvement. The 1 - 60 leveling process (except for the draenei or blood elf starting areas) has long been fraught with serious flaws. Going through it the first time wasn't so bad, since exploring everything felt so new, but doing it the third and fourth times meant sheer boredom. I remember many times going to a zone, completing many or all of the quests there, and leaving without ever feeling as though I had really "been" there. Except for a few real gems, quests mostly involved spending a lot of time running long distances in order to kill more nameless bad guys -- they felt more like pest control than adventure. Just being there seemed to remove me from the story of Azeroth, and dump me in some other world where there was nothing important happening. Vast stretches of land on the Azeroth map meant absolutely nothing to me as a roleplayer: no character, no story, no meaning.