I played WoW for 9 years. God help you if you were a brand new player because you had to pay for every single one of these before you could play an expac....

Original base game
Burning Crusade
Wrath of the Lich King

With their third xpac the finally got smart and you just needed the vanilla game installed and they combined all the xpacs into one BIG game to install lol.

PS: Warlords is losing them subs because of no flying. But I read they are fixing that.