i will go with this to, i know WoW wasnt the first MMO, but its certainly the biggest and pretty much every mmo after it has just followed the same model and same basic classes and sometimes even same abilities with different names, none of them bring something really new and innovative to the genre, most new mmo's just seem to be "wow with newer graphics" but with less content so people dont stick around. the genre has sort of become stale with nothing new added.
also, what i first loved about the mmo games was the community, but thats mostly gone from wow now anyway, i think the a mix of the game being new to me and the great community was what made vanilla/tbc great for me, you just dont get the same feel 7eyars on when its not all new.