World of Warcraft has suffered from dwindling subscriptions in recent months. In fact it went from 11.1 million subscribers to 10.3 million subscribers in three months. Blizzard's John Lagrave commented on the dip in sales and even acknowledged that competing MMO, Star Wars: The Old Repbulic, may have played a part in WoW's decline. (Even with a decline in subs it's still one of the most played games, ever.)
In an interview with Eurogamer, Lagrave said, "Of course people are trying Star Wars - our development team are trying Star Wars! I'm one of the few people who's still playing it actually, but yeah we've seen a dip in subs. It certainly has to at least be attributable to The Old Republic, but it's also attributable to people who want to wait and get Mists of Pandaria, so it's not surprising."
It makes sense that some people took a break from WoW in order to play SWTOR but I would put money on those people coming back for Mists of Pandaria. I personally also know several people who liked SWTOR, but felt PvP wasn't up to par with WoW so they reactivated their WoW accounts.
Are you still playing WoW? If you're not anymore, what does Blizzard need to do to win you back?