I admit, I have no point being in this thread, as I am an even split. I have 4 characters I play about evenly; 2 Alliance 2 Horde (draenei paladin, worgen druid, troll shaman, goblin rogue) and I must say as alt friendly as WoW is these days I thoroughly enjoy having both.
I like seeing both sides of the fence and having the experiences of both factions. Feeling Horde-ish today? Fine, I'll go Horde. And vice-versa and so on and whatnot and whathaveyou. Also I'm a lore nerd and questaholic and I get a little bit of a nerdgasam when there are complimenting quests (like in Grizzly Hills when Horde has a quest to imbue eagles with spiritual sight to observe the Alliance and Alliance has a quest to kill eagles as they're being used as Horde spies).
Although, to be honest, if I had more time in life, the difference would be split Hordeside, as my troll warlock would be the next to receive playtime love. Because trolls.