To answer why is wow so big, it was many peoples first and ONLY mmo, not everyone is a traveller of the mmo-space or as jaded. People stick to what they know/enjoy/get used to.
Yes its also polished, and in some ways does provide a little something for everyone to enjoy.
Nothing else, not even blizzards next mmo will have that affect on the market again, its an outlying statistic, an oddity that wont crop up anymore. It blew the MMO market open as wide as it did all on its own, without ever meaning too.