At its most basic, pushing players through the levels is a matter of character focus. In Mists of Pandaria, WoW turned away from spreading its affections amongst the so-called stable of alts in favor of encouraging a single main character. That's a compelling approach for players like me who enjoy running down the side streets and back alleyways of gameplay in search of every last way to improve a character. Players who leveled up during Blizzard's later era of streamlined leveling, however, are used to cutting a direct line to the finish with an expectation that they'll be able to maintain multiple characters all the way through the endgame.
What do you think about swinging the focus back to a single main character? Do you think bringing alts up to the same level of development as a main character should be easy, making them true alternatives to your primary character? Or should alts remain easy to level but harder to fully round out at 90?