I've been playing WoW since Vanilla...during Vanilla WoW I'd have to say that places were much more exciting, the quests...the travels...and I personally enjoyed Vanilla zones much more than WOTLK and BC. one of the reasons was because there were no flying mounts, you had to mount up and travel from one place to another... or pay a flight master to get you somewhere. with flying mounts you can just mount up and fly wherever you want. is this sort of ease in travel even needed?
and besides...staying up in the sky 24/7 is just silly. that's why I'm making this suggestion thread.
There should be some sort of "fuel" for flying mounts (like a fatigue/breath system) once your flying mount TAKES OFF from the ground you'll see a (Mount Energy) bar which is simmiliar to breath bar on the center of the screen (or under your player frame) your flying mount can stay up to 10-20 mins in the sky without any need to come down and rest.
once your mount comes down the energy bar starts to fill slowly...it may take a full 5 minutes for the energy bar to fill up. which means you need to use your ground mount till the flying mount's energy gets filled (because ground mounts run faster on ground than flying mounts) the mount energy is shared among all flying mounts which means if your Gryphon mount is low on energy...your flying carpet is also low on energy.
this idea also makes the long forgotten ground mounts more useful.