As a former USAF pilot, I think WoW flying is pretty realistic, except for one thing: stopping in mid-air. In WoW you can stop in mid-air, and then go vertically up or down. You can land easily and safely on the tiniest ledge or rock. And that defeats most of the “hazards” in the questing game, which is (in my opinion) why Blizzard doesn’t allow flying in the first 6 months of each expansion.
What if you couldn’t stop in mid-air? How about a goblin glider plus some thrust, and perhaps a minimum running distance to launch, like an airplane? I don’t know if that would avoid Blizzard’s “problem with flying” or not. But it’s worth suggesting. A lot of us players want flying even if we CAN’T use it to make questing/gathering trivially easy.
You can’t do it with all mounts: most of them are birdlike with wings, or magical. It makes sense for them to hover in mid-air. But you could allow goblin gliders with some kind of thrust.