When I first started I rolled Horde simply because when I was 14 I liked things that were evil/bestial/savage/black in my fantasy games. So I played a Undead Mage and had a Tauren Warrior.
4 years later and my taste in fantasy changed a lot and I'm not sure why it did precisely but I began to prefer things that were simply nicer and more tranquil, along with the colour blue . Since then I've really enjoyed the classic fantasy image of valorous Human Knight. The races of the Alliance certainly weren't as original as the concepts for the Horde ones but I certainly felt like they were the classic image that I liked with it's own little Blizzard twist on things. Anything that's Lawful Good.
There are also many other minor tastes that changed just in WoW with me. Stormwind seemed more appealing as it was a very awe inspiring city with epic music and where everything seemed rather tranquil and civilised, whereas Orgrimmar had mud huts and half naked Orcs patrolling it's streets. Alliance music as a whole I just prefer especially a lot of the recent music we've had.