Levelling is the albatross around WoW's neck.
It keeps new players from the end game, and without the end game there's nothing to keep people attached to WoW. It takes so long to get there I've seen people buy the entire set of expansions and quit the game somewhere around Northrend.
WoW is a Theme Park MMO, but it's a theme park at the top of the mountain. And the mountain keeps growing. And the people already in the theme park can't see the problem. Sure, they put in hand rails, but it's still a tedious fucking climb. Back in vanilla, it didn't matter so much. There wasn't much in the theme park anyway, so it was more about the view and the journey. But those days are long gone.
They need to either...
1) Move the theme park so the rollercoasters are spread all over the mountain. Start LFR at level 30 or so, down-level higher level people so queues can put you in random content from all over WoW's history. Gear rewards would be scaled for your current level. All content would become current content.
2) Ditch the climb completely. Everyone starts off at max level, with a tutorial starting area that you can't leave until you've earned all your abilities (like the DK area). Upon leaving you get to explore the world, do PvP or get raiding. Quests would reward last tier LFR level items that scale up so everyone stays on the same footing.