Naturally, one reason is because the bloody early zones are SO STINKIN' LONG. Does anyone else remember the quest in Darkshore where the quest giver (Oro? Some kind of tree spirit thingy? Wears lanterns as earrings?) is at one end of the zone and the quest mobs are at the other?
However, the biggest reason I can't wait for mounts is because I look at my quest log, my character page, my professions screen, whatever, while running..
... and I run into trees. All. The. Time.
"But, Feathers!" I hear you say. "How does getting a mount keep you from running into trees??"
It, ah... doesn't.