We all know the leveling a new character can be a daunting task (as it should be). But with the addition of another another expansion making it a 5th leveling to 90 (soon to be 100) makes for a very lengthy grind that prevents many from not only leveling that alt, BUT prevents many new players from ever getting to see the content that so many of us play the game for. Many new players never make it out of Azeroth simply because of burn out, the idea of being 4 expansions behind, and having to accomplish what they have another 4/5 times. We all have several characters that have never made it through Outlands, or are stuck in Cata still, or even struggling with the snail of leveling that is 89-90 (even after the XP reduction)
What needs to happen is a major overhaul of the leveling path that blizzard has layed out for it. It follows the order of the expansions which is understandable as that is how the content has came out. But for new players most of this content is old, out of order, and can be a mess of information. I propose that leveling should be changed to follow a different structure. Characters should not have to be forced the level through every old expansion. The idea that a player/alt can level through Azeroth then proceeded through 1 or 2 more expansions to get to current leveling content is a more brightly lit path, that still allows for leveling to be that grind with rewards at the end but not be so daunting for new and returning players.
The idea would be for a new character to be able to hit 80 on when they are done with azeroth content. Then have their pick of entering either of the expansions to complete their way through to 90. This would solve some issues such by giving the characters a path as to which content they want to play through. If you leveled a character through MOP already then next time you could choose Burning Crusade..and vise versa.
Azeroth wouldn't take more time to level in this case. Just the Experience, and mob levels would be altered. It would create more play-ability in leveling, and give people leveling the first time leveling to not be so overwhelmed with the thought of leveling through 4/5 more expansions worth of content.