THIS. Warcraft was the BEST designed mmo at the time of its release and they have continued to constantly improve upon that and dispite what the naysayers (Usually of the EVE,WAR,DF,Runescape, fanbot gallery) may say THIS AND THIS ALONE IS WHY WARCRAFT IS THE KING.

I've see loads of BS posts (MMOrpg forums, or Forumfall if your REALLY HORDECORE) that its just Warcrafts advertizing or that everyone dose it because thats what their friends do, that if you play WoW you are fat and live in your moms basement ect.

Warcraft's initial sucess is PARTIALLY related to timing, Warcraft 3's popularity ect but the continued long term sucess is because its is a seriously good game put out by a quality Dev.