tBC was about the Rebels of Warcraft 3, who took a ruined world and shaped it in their images.
WotLK was about the choices that Arthas made in Warcraft 3, and the ultimate un-doing of someone who was so absorbed with The Greater Good that he was tricked into doing the polar opposite.
Cataclysm was about the role of neutrality in a world where tensions from the three wars had begun to rise up again, and how that neutrality was the only way to conquer the forces that had risen up to destroy the world. Thrall was born into a society that hated his race, but he grew up understanding that the two races could and should be able to live together. In other words, Thrall perfectly represented the neutrality element that the story embraced.
Although it is still too early to say for sure, I believe that MoP is about overcoming the mistakes of the past and taking account of your actions. Varian, Thrall, Vol'jin, Jaina, Lor'themar and even the Council of Three Hammers have all gone a long way in this regard.