I can very strongly say after playing every p2p mmo but eve and planetside, that iv'e figured out why wow is so popular.

Simplicity

Why simplicity?
Look at All the MMO's that have come out since 2008 you need a up to date computer to play it.Most people i know have a Old Dell from the dude im getting a dell commercial.Which runs wow fine.
Then we have newer games which like for a person that does absolutely no fancy stuf on your computer (talking a P4 HT with no AGP or PCI-E slot here) they will make your computer explode...

Also the game play is for lack of a better term "Jack of all trades" good at all master of none. PvP is good if you just play for the occasional break in leveling.PvE is pve lol not like grind alot(Aion) or do Quests till your Eyes bleed(Lotro)Raiding is fun on a casual basis,And the crafting isn't mega time consuming(Aion)being Being ganked (asherons call and aion <..>)

Basically its your ultimate Pick up and Play MMO your old grandma can play it, without being super confused.

Dont get me wrong if you past a Casual Gamer like the guys that delete thier Character on youtube no clue why or the dudes on CODMW2 who have prestige rank 10 which is like damn .... its probably gonna get kinda dry and boring after a while.

Just so you know i play Age of Conan and other shooters and my Mother and sister play world of warcraft like a relaxed basis they just hit 80 after 5 months .This is no way shape or form a fanboy post cuz i like pretty much every(P2P) mmo ive ever played , just a opinion.