Alex Ziebart explains:
"The Alliance opening quest in Pandaria involved a strafing run in gyrocopters. You get to a point where you're stuck in a fixed position until you blow up a Horde boat. Blowing it up required you to aim at an explosive barrel and shoot it, but there were so many people doing the quest at once that the barrel didn't respawn fast enough for players to progress. Only one person could finish the quest at a time. It created a horrific bottleneck. As the first hour of the expansion went on, the cloud of Alliance players waiting to shoot that barrel grew large enough that you could no longer see the barrel to even attempt to shoot at it. You didn't even know where you were aiming. The first 2-3 hours of the expansion for the Alliance was sitting in a gyrocopter, firing rockets blindly in hopes of being the person to hit the barrels."
So not only did the Alliance lose Theramore to start the expansion, many of them couldn't even progress past the opening of Mists of Pandaria. What a way to start the show.