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One word: the engine.
Cataclysm, TBC, Vanilla Wow, expansion X in 4 years: it all doesn't matter.
What sets Wow apart is the very specific engine for an MMORPG.
All the rest of fantasy MMO's have a weird handling of their worlds (no background loading) and especially the avatar controls.
The moment a game comes along which will handle the "weight" of its avatars against the back drop of a world, you can expect a competition, until then you'll frustrate your players once they handled Wow's avatars and background loading worlds.
Future designers here is an advice: make a proper developped engine first (no longer working with standard engines), then we can talk again.
Even non MMO's these days use pathetic bad engines (read the newest Fall out). It's as if the characters in these games are crippled and handicapped.
The numbers of pixels don't matter, it is the handling of those pixels in game.
You can't fight decently when you're pulling unresponsive strings.
Most succesful other games with extreme good engines: Mario 3D, Donkey Kong 64. You can add other....