One of the things that has always intrigued me about the World of Warcraft faction division, is that most of the people who play Horde are very, very firmly and patriotically "For the Horde!" Whereas a lot of Alliance are rather less enthusiastic about it-- they like Alliance, yes, but they don't go around posting "FOR THE ALLIANCE" as every other comment in a WoW YouTube movie or proudly proclaiming it on their website, whereas a lot of Hordies, well, do. (Note: I know this isn't everybody... just the majority, in my experience.)
As somebody who really, truly, honestly enjoys playing both factions equally and is actually fond of *gasp* all of the races (even gnomes... actually I have a secret here: I adore gnomes. If gnomes could be hunters I'd never play another race), I've always found it interesting that there is that distinct difference there, and that Horde players get so much more excited about simply being Horde than most Alliance players ever do about being Alliance.
Now, I've heard all the out-of-character and out-of-lore reasons. I've heard people say that they simply like the aesthetics of one over the other, whether it be the look of the races or the look of the places. (Somebody once gave me "Elwynn Forest is a lot prettier than Durotar" as their main reason.)
And yes, I've heard the ever popular "People are more mature/friendly on [insert faction here]." Honestly my views on that last one is that it really comes down to what server you are on, what server type you are playing on, and the people that you encounter, as I have had both very positive and very negative social experiences on both sides of the faction fence. When I worked at a video game store for a few months this past summer, one of my coworkers told me "Alliance has the immature little kids, and Horde has the immature adults, it's just a matter of which ones you'd rather put up with." Now don't get me wrong, I'm not saying I agree with him (I tend to assume the positive about people anyway), but I am saying that he sort of has a point in that you are going to find immaturity and negativity anywhere in the game-- just as you are going to find some very amazing and friendly people anywhere in the game.
So as you can see, my views have long been very neutral and unbiased regarding this issue. So it sort of took me by surprise when I realized over the past few days playing a lot of Horde alts, that Horde does indeed seem to have a homier, cozier feel to it. And I think I may have pinned it down (for me anyway):
I think it's the NPCs and how they react to you, and the things that they say. The Horde NPCs have a very "fatherly" sort of quality to them. Taurens will tell you "be careful" when they put you on a windrider, and they sound so genuine when they say it. Same with the orcs when they tell you to "be safe". Even trolls, when they say "You relax, mon"... yeah, it sounds so funny, but it also really is strangely relaxing. The Horde sounds like they are concerned for you.
The Alliance NPCs are more distant. Friendly, yes, but in a very formal way, especially with the humans and night elves. Dwarves are up-front and happy and tell you to "Keep your feet on the ground!" but it seems more... like a "best buddy" type of thing instead of a warmer familial thing. Really I think the closest the Alliance comes is with the Draenei, which is fitting since an unusually high amount of die-hard Horde players that I've come across admit that they quite like the Draenei. I think that maybe it's because the Draenei has that same caring "aura" that you get with the Horde.
When it comes down to it, it just seems to me that the Horde is really about being "a family" and as this is sort of subconsciously represented throughout the game, it rubs off on the players and the players become a family as well, and this leads to a healthy amount of hometown pride. Alliance isn't so much a family, it's much more independent. I'm not saying that's bad... I'm just saying, the Alliance's faction allegiance is not going to manifest itself quite the same way the Horde's does.
I think I am always going to be a Stormwind girl at heart; it's where I was raised on the game, it's where I've met my best in-game friends, and I simply love the aesthetics of the Alliance locations. And I know it's seeped into my blood, whether I like it or not, because the other day when I saw some guy at work wearing a big Horde emblem on his coat, the very first thought that came into my mind was not "Oh cool, a fellow WoW player", but "Oh crap, it's the enemy!"
But I'll be darned if it doesn't feel good sometimes to log into my hordies and come "home" to the family in Thunder Bluff.*
* I have never really liked Orgrimmar for some reason. When I play Horde, I hang out in Thunder Bluff.