The reason WoW has 7.4 million subscribers is because they decided to overhaul their class system, character models and graphics. WoW also had other cosmetic updates and redesigned their character models during expansion. Also players tend to live in WoW as opposed to other MMORPGS. The one thing Blizzard does with WoW that sets it in another league compared to it's many WoW clones is constant polishing and tweaking of the game. WoW is a game where they would overhaul one class if it would draw more people in.
