The big news from Blizzcon last week was the announcement of World of Warcraft expansion: Warlords of Draenor, and the reveal that this won't be the final expansion. However, according to a post from a World of Warcraft community manager on the official Blizzard forums, the developers "intend to disallow flying while leveling from 90 to 100, and have flying become available again in the first major patch for Warlords of Draenor."
The lengthy post explains that flying while leveling was a special case allowed for Cataclysm since the way zones were mixed together, "it would have been really jarring to dismount you as you fly into Hyjal, etc. but we would have disallowed it for Catalcysm zones as well if there was a reasonable solution there."
The post says that the primary reason for the cut is that "flying trivializes combat... Flying allows you to escape or enter combat at-will. There's a reason why flying isn't allowed in dungeons and raids, or battlegrounds and arenas, and that's because it would trivialize the core mechanic of the game in those areas: combat. For much the same reason it trivializes how content is approached in the outdoor world based on the simple fact that you can lift off and set down wherever you like."
Players will be able to regain the ability to fly in Draenor with the 6.1 patch, but it won't be just a simple unlock you can pay gold for. The post explains, "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. 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."