Without flying, players either travel on the ground or with flight paths/Ogre Waygates, which have fixed locations. And those usually aren't placed next to the Quest objectives, so players still need to move there themselves after landing/zoning in. Where's with flight, players just swoop down the interesting things; rare mobs, rare items, Quest mobs, players,.. and afterwards again hop in and fly to the next target, making everything between those two points... kinda pointless to be even added there since everybody just flies past them.
Yeah, we have had discussions about anti-air cannons, dismounting birds, no-fly bubbles, being unable to fly up in quest area, and such.. but then again, how many would enjoy those? Being dismounted all the time, sitting on parachute and wondering how the life goes while waiting to reach the ground.
It's easier to blame for it, than to make Blizzard make huge rewamps to professions or tags, nobody even wants to group-up since most of them is designed to be killed alone or are just easy anyways, and most of them apparently doesn't want to share the loot. It's bloody annoying when some jerk swoops down as soon as (s)he sees rare marker on the map and tags it just before I'm there due insane speed and approach difference (terrain, canyons, etc..).
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But anyways, it's an experiment which lasts the first major patch. They already said that the situation is about 50/50, and I'm pretty sure that "overwhelming" majority does want flying to be enabled once next patch is released and content is a bit older. Yeah, I'm anti-flier. Yeah, I want flying to be enabled with Patch 6.1 in patch 6.0 areas.
My beloved Crimsy is going to take a small rest,..
http://i.imgur.com/OGXDijL.jpg
...while I'm securing the skies of Draenor on my fearsome mushan.
http://i.imgur.com/32ceP0b.jpg
The skies of Draenor are under control of evil Iron Horde and Arakkoa, and are waiting for their liberator, are YOU up for the task?
http://i.imgur.com/UWSZ8HQ.jpg
http://i.imgur.com/Kod5lH3.jpg
TLDR:
Clear the current content's patch on ground, speed up the process and unlock flight once it's "irrelevant"/"old".