Like I said earlier the Alliance could be breaking the siege. They would land on the beach near Orgrimmar or launch an attack from Ashenvale (revenge of the Night Elves, yay!). The Horde would be sending a unit (the raid group) to weaken the siege. The bosses would be the same, only the trash and some secondary objectives would be different. Also, the Alliance would not be handing back the keys of Orgrimmar to the Horde, since they would already hold them. The Horde is not humiliated to owe the Alliance their capital city and the Alliance would be in good position to negociate some land back from their allies of convenience... Both win!
In the other hand, it would be badass to break through Orgrimmar and kill Garrosh in that throne room... Also, the blues did say that Varian would ask not to kill civilians, which would make no sense if the Alliance is breaking Garrosh's siege, since there would be no civilians there to kill in the first place. Of course that was even before MoP was launched, so they could change that.