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.
I found playing a worgen on the beta to be positively eye-opening in this regard. The story from beginning to end is about unchecked, unbounded Forsaken aggression against a people who had never fought them, never attacked them, never tried to invade Lordaeron or claim lands held by them. The people of Gilneas had gone out of their way to avoid the Forsaken, and it was Sylvanas (and behind her, Garrosh Hellscream) directly attacking a neutral nation purely out of naked lust for conquest and territorial acquisition. By the time you're done playing a worgen through the starting zones, you will be ready to put orc babies on spikes. It may be the first time I've ever felt the factional hatred entirely from the game instead of from other players.
Nowadays, it seems that the balance is pretty even. A great deal of the conflict between Horde and Alliance is set up in the story, but players still take the lead (the point of the spear, so to speak) by keeping said antagonism alive via ganking, world PvP and keeping BGs and zones like Wintergrasp/Tol Barad active. I'm curious to read what you think keeps factionalism in WoW active and if you view it as a positive or negative for the game.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.