I only play Alliance, and I"ve been that way since I started playing at WoW's vanilla release. I started off playing Alliance because that's what my gaming guild was playing. We played on a PvP server so it was easy to start thinking of the Horde as the perpetual enemy. From my perspective, the Alliance were the Good Guys, and the Horde were the Bad Guys.
When I started playing F2P, the lines were more blurred. There wasn't the same world PvP constant apprehension, plus I came to know a fair number of good people through the forums who played on AP Horde side. We would wave and emote at each other when we saw each other in game. Mentally, I changed from a guild-centric mindset to a server-centric mindset. By that time, though, the aesthetic of the Alliance cities and races were fully ingrained in me. I found the Horde races to be ugly, relatively speaking. I've probably made one of each race, and I always try out the emotes for the races to get an idea of what the designers think the race is "about". I ended up playing a goblin and a blood elf to about the mid-teens, just to get the storyline that had been created for them, but still didn't like the aesthetic as much. (For that matter, I also played a worgen to the mid-teens for the story, but also didn't like their aesthetic either.)
Now that trinkets have a 2 minute cooldown and the human racial ability isn't so hot anymore, I might be more inclined to try other races, but I think I would still stick within the Alliance. Gnomes are kind of fun because of escape artist, and the idea of this 2 1/2 foot tall humanoid kicking ass is pretty hilarious to me. I might could see finishing out a goblin character someday, but good lord you guys's cities are needlessly complex to navigate!
