Spoilers for Cataclysm in this post, me hearties.
One of the things I've noticed in my time back-and-forthing between Alliance and Horde toons is that each faction seems to have many, many vocal partisans who believe the opposite faction to be filled to the brim with churls, knaves and scalawags. Perhaps even hooligans and ne'er-do-wells. You see it all the time in general chat: "Those ally fellows are nothing but mountebanks!" Or perhaps: "Horde? Nothing but disreputable scoundrels, bounders and cads!"
What I find interesting is how much of this factional divide is created by the game itself ... how much comes from quest lines and zone design and world events ... and how much is purely based on the players. It's true that over the past several expansions, we've seen a shift in the game itself from the days of the AQ gate event and the opening of the Dark Portal, when Horde and Alliance stood shoulder-to-shoulder against threats to Azeroth, to the present Wrath/Cataclysm direction when even Old Gods, Lich Kings and insane dragon aspects can't get the Horde and Alliance to cooperate.
Still, until fairly recently, I'd never really felt much of the infamous Horde/Alliance hatred from the game itself. Even the Wrathgate /Battle for Undercity and the Broken Front quests didn't come close to matching the intensity of a forum flame war or a really acrimonious Wintergrasp battle. Playing Horde back in the day, you couldn't help but notice the perception of superiority Horde players (including myself, at the time) felt over Alliance. But the story didn't really have much to do with it.