When I watch random WoW movies I always feel a tinge of sadness when I see dancing Undead, cheering Belves, and Tauren doing the peanut butter 'n jelly. I started out playing Alliance, but after thirty levels or so I joined some friends rolling Horde, so I made (a bunch of low level alts and) a priest. She was undead, and I loved her.
Sometime thereafter we decided to make new characters on Alliance side on a new server. I wouldn't have known it then but the spacegoat paladin I created to run around Westfall (she thought she was human, and went straight to Stormwind upon her creation) ended up being my main for most of Wrath. Alliance is okay. I don't have anything against them.
It's just that the Horde is so much cooler. I mean, really, so much cooler. Alliance has better choices for Paladins (and by better, I do mean, they have a choice while Horde does not), but, which warrior strikes more fear into your heart: an orc or a gnome? Gnomes are cute, but, if I wanna be a fearsome warrior, I wanna be a Horde.
Blizzard has made switching factions and servers so much easier now, for a price. I did server/faction transfer my Druid on the Horde side to join my raiding characters on the Alliance. I swore it would be the last time I did it - the costs get ridiculous for multiple transfers/changes. When I made that choice, I decided that Malfurion is where I was going to stay. Now I have three 80's, and keeping them together to share crafting for raiding is really what I want. Our friends have called us a one-stop raiding shop, because between Mayhem and I we have everything we could possibly need to raid - enchants, jewels, armor kits, spellthreads, flasks, drums, glyphs, and scrolls - all ready just by logging in to one our characters, most likely perched at the Eventide. I really like not having to rely on others - and I enjoy being prepared. I carried around a Scroll of Enchant Shield for months just to be ready for that moment when I replaced my Naxx shield. (And believe me, it was soooo worth it).
Now if only we could get Replenishment on a piece of paper, I would be a very happy Paladin.
Anyways, the point is, I know enough about myself to know what things I want to spend time doing in the game. I don't have the time to keep up two mains in different raiding guilds, for example, while I do have time for one main and an alt (maybe even two). I got my Druid up to speed and ready to raid ICC in no time. I stopped actively seeking upgrades on her because I can pug ICC on her whenever I want while still pulling my weight in a group. She doesn't need "more loot" in this expansion. I'll take an upgrade if I can get it but I don't spend time worrying about her stats like I do my main (more haste! more int! more sockets!).
With this in mind, I play Alliance because it's where I am. I don't think I could fit both factions into my playtime. Nor do I want to try. The disadvantages of playing cross-faction (from the crafting/sharing standpoint) are too annoying. And like I said, I don't have time for two mains, so why would I create an lonely alt who can't share in the resources I already have, knowing I'm not going to devote enough time to it anyways?
And yet, I still would rather be identified as Horde. Their slogans are better. Blood and thunder! Lok'tar, friend! FOR THE HORDE! Their dances are better. Sure, the human males know how to deal with Disco Fever, but the undead males know how to seriously rock out. Even the capitols - Stormwind City is alright, but fuedal and prissy. I'd much rather hail from Orgrimmar, with its huge bonfires and carved wooden totems.
It's merely aesthetic differences for players focused mostly in raiding as I am. Inside the raid, there's few, if any, differences. The racial abilities our characters have are nice but in no way make a class viable or inviable. There is no playstyle change between factions because our playstyles are built around our gear, talents, and class choices - not factions. The people you play with shape your end-game PVE experience more than your chosen faction.
That doesn't stop me from wishing, once in a while, that my priest was undead (or that my paladin could be undead). Hell, I'd rather call myself a blood elf before I admitted I played a night elf (druids excepted). Sometimes, I just wanna be a Horde.