Yet interestingly enough only 30% of people make it past level 10 and keep on playing the game according to Blizzard CEO Mike Morhaime. (http://www.escapistmagazine.com/news/view/98245-Only-30-Perc...). Also related to this is the issue that WoW seems to have peaked in its growth.

So even Blizzard has a lot to learn in this regard.