mostly looks & lore reasons; I have two characters
-Tauren female Shaman...I just really love their casting animation, male are way too bulky and ugly.
-Human female Paladin...I am a big fan of the whole Argent Dawn lore.
I always liked the Shaman/Paladin aspect in the game and how they sometimes have similar spells but with different effects: Totems vs. Blessings, etc.
So my Shaman is Horde, my Paladin Alliance
Strangly enough, I only choose vanilla races to play. I don't like the newer races, they destroy my personal 'wow-core-feeling'