Actually, Deathwing already ahd a history in the lore, and at least one book. Deathwing had gone into hiding, and his dwarven army forged him armor to protect and hold his body together. Thus the new look.
Blizzard decided that they couldn't add anything to the world as it was, so they changed it. Too bad they've been nerfing things to lower levels, removing whole things like the Path of the Titans, and practically giving away event items like the Ashbear to those who want it. It's losing its epic flare.
And for Deathwing, I've seen comics (particularly Wow, eh?, Sara & Kleeyo, and Complex Actions) make him into such a wimpy or scared character, it's barely funny. No longer does the fierce, scary, earth shaking dragon exist.
And the Cataclysm menu screen! It's horrible!