Just blue-tagging this for visibility. Thanks to Bashiok for the post below from the US forums :)
I just want to emphasise to those of you who are worried: we're working to make sure you won't be hugely inconvenienced by the loss of flying as you explore the new content. There'll also be a sturdy flight path network to get you from A to B.
So a couple things I wanted to bring up in addition to what I posted earlier. The first was that we're of course refining our questing experience, you may have heard a bit about it from BlizzCon, and that expands to max level content as well. In Mists of Pandaria there was quite a bit of outdoor content at max level, but it was almost entirely in the form of dailies. From BlizzCon we shared we're looking to provide something quite a bit more story driven like the 5.1 Operation Shieldwall/Dominance Offensive with some of the exploration lessons we learned from the Timeless Isle, and that extends to the max level experience as well. We want the transition at max level not to be questing and story to a harsh "now you do dailies", but something that's more natural and feels a lot like the leveling experience just at max level for better reward. And so that goes back to all of the reasons I gave before about how flight interrupts some of the core mechanics we think present the best leveling and questing experience, and I thought it'd be good to point out that it'll also play a big role at max level due to our questing refinements.
The second thing is we really like when flying feels like a reward and something you've earned for each new expansion or area. It's such a powerful ability and we look back at BC (at least for epic riding) or Wrath and those were really big keystone achievements for us as players (mainly because they had some pretty huge gold costs on them...). Regardless of the unlock method though it was one of the bigger moments when you unlocked flying, took to the skies, and were able to see the world from an entirely different vantage because for most of us it wasn't easily attainable. Our plan has been to recapture some of that but make it a bit cooler, and while we don't have specifics just yet, it'll be something more involved than just paying gold; something that further emphasizes how powerful flight is. Unlocking it won't just be level 100 on the day 6.1 comes out with some gold, but something you'll be working toward over the course of the initial expansion release, and 6.1 will introduce the final steps to complete it. Think of something like a mini Legendary cloak quest. It's such a enormous power to be able to fly we really want it to be more of a key moment when you unlock it, and something that feels like an accomplishment more than just shelling out some gold like we've had in the past.