The WoW general forums have been on a bit of a nostalgia kick lately. First, someone posted old maps of Azeroth from WC1 and 2 for discussion. Now, Aest of Twisting Nether asks, "If somehow, the Horde and Alliance broke out into all out war, who would be left standing?" He also mentions that the Burning Legion, the Illidari and the Scourge have all taken a holiday, so they can't interfere.
Of course, the first time this happened, the humans banded together and took down the orcs after losing Stormwind. But like Aest mentions, things are different now. The Horde has been on a consistent upswing since the Second War, and is strong and united under the leadership of Thrall, whereas the Alliance has lost High Elves and have a very divided high command. Sure, the Blood Elves and the Forsaken are in the Horde less for "Yay, Horde" and more for "Let's get revenge on our enemies", but that works okay, especially since the Forsaken are in the weapons of mass destruction business.