I finally hit level 70 while gaming casually. Subsequently I have also purchased my flying mount and final skills. Now, I can think of a few reasons why flying mounts are not allowed in Azeroth but I really haven't figured out the 'real' reason why they are not allowing flying pimped-out rides on the other side of outland. Personally, I would feel a bigger sense of achievement if I were allowed to take my new mount out into the world when I am helping my friends with their primes and alts out in the old world. I really see no harm in letting someone use their flying mount out in the open.
The few restrictions I would place would be not allowing someone to fly too far out from shore, put the same fatigue rules out there to prevent people trying to go from continent to continent with their mount. You know some idiot will try it and complain when he falls out of the sky mid-flight during the continent change and looses his body due to fatigue. I would also put up some sort of invisible barrier around places you don't want someone to fly. (Though I can't think of any right now.)
They've already covered the whole 'flying over the oposite faction's city marks you and gets you killed quick' thing. Other than that I can't figure out why, why blizzard can't I use my pimped out ride in Azeroth?
Edit: In hindsight I suppose sloppy terrain mapping and oddball programming would be the real reason. While in the beta I swam my undead warlock from the undead starting area all the way to westfall along the outside of the mountain ranges just to see what would happen. It worked. On the outside are nooks, crannies and trees and shrubbery growing in odd places. I guess they'd have to fix everything in azeroth and they just don't want to spend the man-hours doing it.