For background, please read Talent Trees.
Leveling in Mists of Pandaria is definitely missing something. There are too many dead levels, levels where nothing interesting happens. As well, there is a distinct lack of choice, of the feeling that you are crafting your character as you level. In previous expansions, the mechanic that accomplished this were the talent trees. Every level or so, you got a talent point, you had a choice where to place that talent point, and you slowly improved your character.
Now, from a endgame point of view, the talent tree system wasn't very good. There was no real choice, and all comparisons being implicit made it hard to offer meaningful options. The current explicit choice system is much more interesting at endgame.
But the current system is much less interesting while leveling. You only get a choice every 15 levels, which is an eternity. Often, the choice isn't really relevant to leveling gameplay. For the purposes of leveling alone, talent trees were a superior mechanic.
Here's my proposal: Bring back talent trees, but have them only be used for leveling.
Have one tree structure for each spec. Each spec tree contains all the active and passive abilities that a class would get while leveling. But each tree consists of exactly 81 points. With a point per level after 10, that means a max level character has filled out the tree entirely.
Thus talent trees become a leveling mechanic only. It becomes the way abilities are delivered while leveling, rather than automatically gaining those abilities. Players can choose the path through the trees, making a difference in how you choose to level. Maybe you'll get Holy Radiance early, maybe you'll make a run at the cooldowns, or go for important passives first. While leveling, the order in which you gain abilities is important. But it's still a temporary choice, because leveling is a temporary state.
Since all talents will eventually be taken, the design of the tree can be as intricate as you desire. You can have lots of prerequisites, multiple paths, even arrange the tree to form a pretty picture.
At max level, you can just assume the tree is filled in. Switch specs, and the new tree is already filled in. A max level character has all the abilities she requires for endgame, just like the current system for Mists.
Talent trees would make leveling a little more interesting. It would add some more choice back into the early game, and allow you some agency over how your character developed.