well i mean it should be pretty obvious from the start, they even stated when tbc came out that "azeroth was not designed for flying mounts"
there are even zones that aren't even made yet, gilneas south of silverpine is a good example.
though "when" this change occurs, and (assuming here, it should be inevitable) when the maelstorm is ready for exploring, it would be nice to have like the ability to fly literally all over the planet, sure the continents would be seperated by load times, but it would be pretty neat to fly from say ironforge to darnassus.