Making the world virtually worthless and nothing more than a hub to enter isolated instances makes the game feel empty more than any flying mount ever has. Not that flying doesn't hurt it any, but... the world being almost entirely worthless hurts the world most of all, and it always has. As the world has been pretty damn worthless for a really, really, really long time.
People just like their easy scapegoats and/or mad about not being able to gank people. Instancing does way more harm to an MMORPG than anything, if you value an actual world with tons of players in it, IMHO. No one ever complains about it, though. Because for some bizarre reason, they prefer it that way. But lord, it sure is anything and everything else's fault.
The fact that people want flying mounts mostly to AVOID the world should be telling to most. Most of the world is so dull and unfun to be in. At least that's how I see it.