Some players, especially the ones who haven’t played is a while, received an interesting email from Blizzard. Looks like that if they are coming back to WoW their account is going to be upgraded to Mists of Pandaria for free!
This is how that email looks like:
We don’t know how is this going to work exactly nor what accounts are going to be affected by this. I’m pretty sure we’ll have an official announcement soon enough, beside this email.
Even if you didn’t receive an email like this you could take a look to your battle.net account and see if anything changed. Receiving all the expansion except for the Warlords of Draenor one for free only for re-subscribing is a nice deal. So if you plan to come back to WoW you could give this offer a shot.