From the poll that High General Turalyon made about Flying mounts (link) it's pretty clear that people don't want to abandon their flying mounts.
Now I ask this:
What could be your reaction if Blizzard introduces a "Flying Fatigue"?
You can fly normally, but when flying, your mount will stack fatigue 'till the moment it's full. At this point it will start to (slowly) fall down and land.
Then you have to wait until your mount gains enough fatigue.
I think this change could bring back what Turalyon wants: (or what I think he wants) a world with a geography that matters.