The Alliance is held together by some very old ties. The humans, gnomes, and dwarves have fought together for a long time. Generations. As much as I know there were ties even before the Orcish invasion.
In contrast the New Horde is young. The Orcs have barely been free of the demons. The trolls and tauren are about as recent. The undead even sooner. And the belfs, they got here yesterday it seems. There hasn't been time for alliances to be tradition and unquestionable, to be taken for granted and unshakable.
Really it is Thrall and his relationships which keep the Horde together. And safe. It was who brought in the Darkspear and the Tauren. While they swore loyalty to the Horde, at the time he was the Horde. With Garrosh replacing Thrall, a lot of loyalty will be lost. Not a split yet... until Garrosh starts doing what he does: run his mouth and start conflict.
As Garrish turns the orcs towards expansion against the night elves and away from their allies, the Horde will fragment. How much? I don't know. But I think it's high time we had a civil war to sort out some bad elements.
I can see it now: the trolls break away, at least some towns, and end up in conflict with orcs attempting to retake the territory. Quests on both sides would set Horde players against breakaway or loyalist NPCs. Eventually it is resolved, but tensions run high.
Two individuals are ultimately killed or dethroned: Magatha Grimtotem and Garrosh Hellscream. While the tauren consider breaking away, she takes the lead with her Grimtotem tribe. A combination of loyalist tauren and orcs put down the rebellion and kill her. Or do we? I see an opportunity for her to have a mere setback and end up with the quilboar, undead or otherwise, along with the renegade Forsaken who have previously been plotting with her.
Garrosh will meet his undoing at the hands of Saurfang, making good on his promise: "I will kill you before I let you take the horde down that dark path again". He might not kill him, but instead beat him into submission and take control of the orcs and reunify the Horde.
I think it would be for the good of the Horde. The tauren and trolls need a test of loyalty, and proof that they can and should be loyal. If they can stay without Thrall, that is the proof. The orcs need a cleansing, a removing of the reckless ones, and a civil war would reveal and kill them. Thrall somewhat kept them in line, but I believe Saurfang can be more direct. He was there and knows what the bloodlust leads to, like a former addict who can explain that actually no, slaughtering men, women, and children is not glorious.
Imagine a Horde which is formed not by murloc attacks and centaur marauders, but by removing all the dissidents: the Grimtotem and Zalzane and Garrosh. Imagine a Horde which is together because it chose, at cost of blood, to remain together. It would not be a pacifist Horde, it would not be a warmongering Horde, but it would be a Horde which would do whatever it needed to survive.
Meanwhile the Alliance can rebuild itself and take some of what it has lost: Gnomeregan, Stromgarde, and who knows what else.
Two factions, consolidating their strength, preparing for war over what is left of their world. My axes needs sharpening...