@Al,

Past bias is in fact the cause of the current Horde ascendancy: when WoW was being prepped way back in the day, Alliance areas were first and most extensively developed, partly due to deadline pressures, and also due to the feeling at Blizzard that the Alliance would be the most popular faction (which they were, at first, by far). The result was a game experience that was very imbalances, particularly in terms of story arcs and questing areas.

Because of this, when the time came, with Cataclysm, to redesign the leveling arcs, the zones had to be redistributed more equitably so that there would be enough Horde zones to work with. This effectively meant several formerly neutral zones going to the Horde. Blizzard accomplished this within lore by building a storyline of an aggressive Horde on the attack.

That has been accomplished, and the designers have hinted pretty strongly that the next step involves the Alliance, reacting to an attack on Theramore, uniting strongly behind Varian Wrynn and going on offense against a divided Horde.

The storyline is ongoing. For a long time, the alliance was ascendant. Now the Horde is. It looks like soon it will go the other way. But if you can't handle your side ever being on the ropes, it might be tough to take.

I play both sides, though my main is Horde. I didn't get upset in Vanilla, when the central story lines all came out of the Alliance (why was I killing Onyxia, again?). I didn't get upset in Wrath, when all the major npcs were Alliance (Bolvar) or former Alliance (Arthas, Tirion). Like I said, it's a pendulum, and I enjoy having ups and downs.