Aeshi said:
Gotta say it's kinda impressive that an engine as old as WoW's can still have that kind of graphical performance.
Every expansion they overhaul the engine. It is definitely not even close to the original engine. There was a slight improvement in the Burning Crusade. They added more lighting effects and the phase technology in Wrath. The biggest overhaul was in Cataclysm when the completely overhauled the graphics and lighting systems, beefed up the amount of polys and the resolution and included more maps to the graphics. Which the biggest change was the water rendering.
Cataclysm may have worse in the gameplay and story sections, however it was a greatly improved engine. They kept pushing the engine throughout Cataclysm, with the last improvements to the sky boxes, as seen in Dragon Soul dungeons and Raid. A lot of vanilla was remodeled and re-textured for Cataclysm
Most of the improvements in Mists of Pandaria and Warlords revolve around their terrain creation tools. Most of the problems with WoW is because most of the models are outdated, not just the character models, but even the terrain and objects were built for the older engines. It would be to much work to remodel everything at once. I believe Warlords was originally planned to revamp Outlands, just like Cataclysm did with vanilla. However after the mistakes of Cataclysm, I think they choose to start from scratch, hence going back in time.