There's no real way to discuss this without sounding critical, which isn't my intent, so let me try and front load this one: I believe that Mists of Pandaria may be the first expansion to end, not with a feeling of victory, so much as a feeling of exhaustion. And I believe this to have been a deliberate choice on Blizzard's part, and moreover, I believe it to be the correct choice, because it highlights the ultimate futility of the Horde/Alliance conflict as the main conflict of the World of Warcraft MMO. There can be no ultimate winner as long as the Warcraft setting continues to exist, only a series of temporary highs and lows as each trades blows.
In The Burning Crusade, we ultimately defeated Illidan Stormrage (who, I admit, I was never really clear on the reasons why we were fighting him -- yes, he could have eventually been a threat, but he didn't actually seem all that interested in fighting us) and Kil'Jaeden, a much more real and present danger whose defeat saved all of Azeroth from a repeat of the Burning Legion invasion. In Wrath of the Lich King, we defeated the Lich King, slaying Arthas Menethil and halting the Scourge. In Cataclysm, we defeated Deathwing, balking his plans for annihilating the world entire. These felt like victories. Whether you were Horde or Alliance, you could feel pride in your efforts to hold back the shadow of destruction and save everything you knew from a grisly end.
At the end of the Siege of Orgrimmar, yes, we've toppled a tyrant. But for Horde players, it comes at a monumental cost - the guilt of being complicit in Hellscream's actions, the trauma of having to have raised arms against one's own faction and storm your own city, deaths of those you've known and even liked on the other side of the civil war (Alas, poor Nazgrim, and flights of wolfriders bear thee to thy pyre) and having to see what depths a former hero of the Horde could sink to, throwing away his father's weapon and his father's final legacy in favor of the same naked grasping for power that damned the orcish people once before. And Alliance players? Well, we get to fix the Horde's mess.
On the one hand, it is a victory, and a decisive one. Both factions made a statement with their role in the siege. Both can be proud of what they've done. But can it be called triumphant? I submit that it cannot, that it is no triumph, merely an end to madness.