it's based on the Warcraft universe and have similar lore (Story) behind it.
By those who don't play it, it is known as a Social life killer that only nerds play.
Those who play it mostly thinks it's a great way to socialize with even more people then you could outside the game. This game have over 11 million active accounts!
Those who used to play and quit, calls it boring and stupid, and often refers to "vanilla" as being much better. in some cases they reject it completly and make fun of people playing it, just to come back and get popular with those who haven't played it at all.