Okay, it's time to come clean. One of the writers here at WoW Insider biased. He prefers one of the factions in World of Warcraft over the other. He's tried both factions... has 70's in both... and has played out most of the quest content and raided most of the 60 and 70 raids both Horde and Alliance side, and in the end, he's come to one conclusion.
I've no doubt fooled all of you with my cunning use of the second person pronoun here (note: it was third person, genius), and... I didn't? You all knew I was talking about myself? Dang. Well, on with it: while I love my tauren warrior and have a blast playing my orc shaman, I really prefer Alliance over Horde. I personally have no difficulty leveling on one side or the other of the factional wall, but in general, I find the Horde quests can veer into unpleasant, even outright evil extremes that I don't like participating in. What's funny about this is, aside from a few quests I really don't like, I can't find any functional reason for my preference. I have met and played with good people on both factions, which is why I leveled two toons to 70 and another into his 60's over there (Consummate Vees, woo hoo!) and I really can't say that I prefer one faction's cities over the other. My two favorite races in the game are Tauren and Draenei, with Orcs and Humans as my second choices partially out of tradition and partially for stature reasons since I play warriors a lot.