Or we come back to my theory of burnout: The older a game gets, the more awesome stuff the game company needs to add, only to just keep the interest level constant. Mists of Pandaria is maybe not so remarkable for what it does, but for promising already a lot and it still not being sufficient to keep the interest up.
To test that theory I thought of a thought-experiment, which for once isn't completely unthinkable: What if Blizzard introduced "classic servers" to World of Warcraft, which would work exactly like WoW did in 2004/2005, maybe minus some bugs and plus some UI improvements? Would that bring the lost millions of subscribers back?
Personally I don't think so. I, for one, wouldn't be terribly interested in playing on a classic server. Been there, done that. While I do consider that classic servers would be a good idea, I don't think that there are millions of players out there who would prefer classic over the current version. Maybe the WoW expansions were too slow, and maybe they never brought us everything we would have wished for, but I don't think World of Warcraft would have survived if it was still on vanilla with no expansions at all. So it is hard to argue that the expansions made the game worse, they only maybe failed to keep up with our growing demands and expectations.
In another, more fictional thought experiment, I was considering a World of Warcraft which got the Burning Crusade expansion in 2005, Wrath of the Lich King in 2006, Cataclysm in 2007, and Mists of Pandaria in 2008. I am pretty certain that in that case Mists of Pandaria would have received a warmer welcome. Four years later it appears more like being too little, too late. Everquest is about to launch its 18th expansion in 12 years, which is probably closer to what players demand. And that still didn't prevent people from burning out.
The alternative might be a MMORPG which resets regularly. Which might be the appropriate point to tell you that A Tale in the Desert is currently in the final events of the fifth telling, and will start over from scratch (with improvements) on December 3rd.