Blizzard is being awfully reticent to say how flying will work in the next expansion. This is leading some people to speculate that we may not be able to fly at all in the expansion.
I thought it might be instructive to look back at the past to see how flying was handled.
Burning Crusade - No flying while leveling. Able to fly at max level. No flying in Sunwell Plateau.
Wrath of the Lich King - No flying while leveling. Able to fly at max level. No flying in PvP zone.
Cataclysm - Can fly any time. Old world remade to allow flying. No flying in PvP zone.
Mists of Pandaria - No flying while leveling. Able to fly at max level. No flying in Isle of Thunder or Timeless Isle.
Mists is actually more interesting than that. Ignore leveling, and look at where max level characters are supposed to spend their time outside instanced content.
5.0 - Dailies across the continent. Flying allowed.
5.1 - Karasang Wilds. Flying allowed.
5.2 - Isle of Thunder. No flying.
5.3 - Battlefield Barrens. Flying allowed.
5.4 - Timeless Isle. No flying.
I think Blizzard has decided that these "max level areas" are better off without flying. And it's hard to argue with that. Isle of Thunder and Timeless Isle were the best max level areas in the expansion.
Then add to that a new PvP zone, which traditionally does not allow flying, and a problem begins to emerge. If Blizzard allows flight in WoD, they have to sequester off the new max level areas. They can't use the main area of the continent. Instead we'll get something like:
6.1 - Isle of No Flying 1
6.2 - Isle of No Flying 2
6.3 - Isle of No Flying 3
6.4 - Isle of No Flying 4
All connected to the main continent by portals or similar.
So I think Blizzard is at something of an impasse here if they want to avoid this. Either make substandard max level areas that allow flying or disallow flying on the new continent. Disallowing flying will make a lot of people very unhappy in the immediate term.
I think Blizzard's best strategy would be to say that there will be no flying until 6.3. Then in 6.1 and 6.2, they can use the continent for max level areas. In 6.3 and up, they can make new content be on the islands of no flying, and allow you to fly in the old max level areas.
Giving a definite time after which flying will resume will quiet the fears that flying will be taken away. It probably won't stop the complaints, but I think that most people will be willing to settle down and wait for it.