The new zones are rebuilt from the ground up and look completely different. Tanaan Jungle (Hellfire Peninsula) is a jungle, Zangarmarsh is a sea, Shadowmoon Valley is beautful and lush, etc.
The new models have over 10x the polygons of the old ones, far more sophisticated rigging, and 2x larger, non-symetrical textures. There's more detail in that new gnome's face than their was in the old gnome's entire model.