I assumed that people rolls their eyes at the Alliance because it's a fantasy game, there isnt much of a reason someone plays it just to be a Human, compared to the Horde, the latter is more monsterous yet has their own nobelness.....then again, people hates Blood Elves for being effeminate...besides the Alliance has Elves, Horde players hates BElves beause its sort of like a guy listening to rap music, getting the lingo wrong, looking like a poser.
Personally out of both factions, i hate Tauren, Blood Elves, Draenei and Dwarves...
Tauren are annoying, have you seen how many Shamen kills you in Battlegrounds? Blood Elves are annoying ebcause they constantly use silence to ruin us mages...Draenei has hot voices, but their Sizes makes it annoying when trying to access small doorways even on small mounts like mechanostriders.
Not sure why i hate Dwarves, guess they're the 2nd alliance race i listed just to balance out the hate.
Like the looks of Gnomes, Nelfs and Trolls though.
Play as a Human Mage though....so you probally know the reason behind the whole shamen, belf and rogue hate..we're just too flimsy against practically everyone.
EDIT: Like the looks of Goblins too, they're basically cooler Gnomes.