I think this is a marvelous idea.
I'm hardly going to pretend that I don't have a stable of rare and exotic flying mounts that I use to zoom all over the World of Warcraft, because I totally do. I use them pretty constantly. The only time I don't use one, in fact, is when I use a flight point because I want to go AFK while I travel, because in the years since flying was first introduced the wonder of exploration has been totally removed from the game. I almost never worry about navigating in the world. There are no hazards I can't simply fly over, and only rarely has Blizzard introduced flying hazards that could dismount me and thus make flying a dangerous proposition. Back in BC, there were some quest zones that had flying mobs that could dismount me or anti-air attacks that needed quick flying to avoid, but since then these simply haven't been much of an issue. Flying made the world safe, and thus, not particularly exciting in the same way that stealth does - exploration without risk isn't truly exploration at all.
But while stealth is a class feature, meaning that it only affects those who chose one of the two classes that can do it, flying mounts are universal. Anyone can get one. And the ultimate appeal of a flying mount in many cases is purely that it allows you to skip the game world, and thus, the game. That is why I believe Blizzard is not only right in this approach, but they haven't take it far enough. They should simply come out now and say there will be no flying in Draenor. And then they should stick to it.
Now, since they've used flying mounts as rewards and special drops for years now, I do think that the technology that allows us to mount those flying mounts even when they won't fly needs to be implemented - even if it can't walk and just has to hover there. I didn't get my Mim's Head so that I would have to walk places. If people have a cool mount, they should get to show it off even if they can't fly on it.
My problem is with the fact that ever since Wrath of the Lich King decided to let alts fly at lower levels than max (in fact, you needed flight to traverse the Storm Peaks, Sholozar and Icecrown at all, really, the first zones really designed with flight as such an important feature that you could even buy a temporary flying mount if you didn't have one) people have used it to essentially evade the game. Efficiency often trumps experiencing the game at all. There was a time (before flight) when people would go out of their way to find and explore out of the way places in Azeroth, and these places (like the strange farms and 'road to nowhere' in the Wetlands) required significant player effort to get to. Now of course that era is gone - there's effectively nowhere on the map you can't just fly to, and thus, nowhere on the map you particularly feel any pressing need to go unless there's a quest hub or things to kill for said quest hub. I never actually explored the Vale of Eternal Blossoms. I got in, got flying, and began zipping back and forth to do Golden Lotus quests without looking around at how the zone connected.
Be honest. You totally did too.
I think designing Draenor without flight in mind and then not allowing us to fly in it makes perfect sense. If you want a lore reason, here's one - the arakkoa can fly and pretty much dominate the skies. Put in a couple of quests where flocks of arakkoa attack Alliance or Horde flyers alongside with huge flying beasties and knock them out of the sky, causing massive casualties. In fact, I'd actually prefer that be the means by which we don't fly - I'd find it hilarious if, instead of permanently banning flight, Blizzard allowed us to do it in patch 6.1, but any flight more than a couple of minutes would attract waves of flying monsters under arakkoa control and you would get dismounted and potentially eaten. How many times would players stubbornly continue to mount up, only to fall screaming to their deaths? I don't know, but I'm keen to find out.
I doubt Blizzard is that hilariously cruel, though. And it's probably for the best.
A world without flying means a world we move in. It's a world where, if you want to see what's over that hill, you have to actually move through what's between you and that hill to find out. You end up seeing and hearing and fighting your way through a more organic setting. Places like the Dread Wastes are significantly more interesting (and in some places, dangerous) when you can't simply fly over everything, which is why I prefer to hit 90 inside the Dread Wastes so that I've explored most of it on the ground. An entire continent where you never graduate from the world is in my opinion the best idea Blizzard has ever had.