The big news in leveling is that WoD will give every account one free level 90 (I gather you can boost any one character to 90), allowing them to completely bypass the old leveling game.
I have mixed feelings about this. On the one hand, if you want to play with your friends, 90 levels is a pretty huge barrier. Plus, your friends can help you adjust to the flood of abilities at 90, not to mention that you can play with Proving Grounds and the 10 levels of Draenor.
On the other hand, I believe that a game should not offer a shortcut for it's "central fun", and leveling is certainly a very important part of the WoW experience.
On the third hand, leveling has been sped up and simplified so much that it is a hollow shell of the leveling experience in Vanilla/TBC. There's really two opposing viewpoints here. One side feels that leveling should take time and effort. The other side believes that leveling is an inconvenience before getting to endgame. In the past, Blizzard has attempted to split the difference, making you level, but increasing the leveling speed so that you reach endgame fairly quickly.
I don't think this compromise has satisfied either side. One side is unhappy that they are outleveling their quests, while the other side is unhappy that they have to level at all.
I hope that Blizzard takes this opportunity to recalibrate leveling to something more sane. Something that may take longer overall, but the pacing of individual zones more closely matches the older experience. After all, everyone will already have at least one 90 that they can use for endgame. And probably this will get offered as a character service for an extra payment.
Then you can skip it leveling you want. But if you want to level, you get a proper, non-rushed experience.