First of all, I personally love flying mounts in WoW but I think flying at max level is too easily obtainable. Do you remember days back in TBC? You had do farm your gold FOR MONTHS to learn flying (EDIT: I meant 280% speed flying) and to buy your first mount. And yeah, when you've finally bought it, you felt like you achieved something big. And now in MoP all you have to do is to get your character to 90th level, and then buy "flying in pandaria" for a very cheap price. That's not how it should work.
So here's my idea:
Let's say, there will be a lot of factions in the next expansion (just like in MoP). To raise your reputation with those factions of course you would have to do dailies or events (like in BF: Barrens or Timeless Isle). But those activities would also provide something like timeless coins. And to learn flying in the continent that will be added in the next expansion, you would have to pay a lot of those timeless coins. I mean really... A LOT. A grind would take maybe around few months but then you would again feel that you've achieved something - after all this hard work, now you can fly on your mount on the new continent.
And the reason why I think flying should be avalaible for currency similiar to timeless coins is because nowadays there is a lot of rich people in WoW so if flying would cost for example 50k gold, it wouldn't be any problem for rich people to buy it so instead they would have to do content and buy it for currency like timeless coins.
Discuss this idea.
I've also made the same topic on the official WoW forums, so if you like this idea, also post there so maybe someone from Blizzard will see it: http://eu.battle.net/wow/en/forum/to...44394?page=1#0
EDIT: It looks like flying in the next expansion is going to be delayed until first major patch - 6.1 and it's going to be obtained after a long chain of quests. Thanks Blizzard for learning from your mistakes!