there's definitely friction going on within the faction though and it is seeming to be between the forsaken and the Orcs.. The Tauren seem to be pacifists in this whole thing. (TBH I done even really understand why they are Horde only real reason i can think is the Human's liking for cheeseburgers.. but I digress)
I kinda get the feeling that Blizzard put Garrosh in there because they hope to engineer some sort of huge faction uniting shake-up at some point and Thrall is just too respected for that to happen to him.