My response, and I should have clarified this better in my response, is that leveling used to be decently fun (as fun as something can be the 20th+ time you are doing it, I mean). It didn't feel overly burdensome because it was, at least, pretty fast. So yeah, the content might be old and tired for some, but it would go by quick enough that it wasn't terrible. Coupled with the level scaling of 7.3.5, the "finishing a zone before the story" issues could be overcome while leaving total leveling as a pace that most would at least be, for the most part, accepting of.
The problem is, leveling in a game like WoW will never be considered "fun" by everyone for the sheer fact that the game has been out so long that most people playing since the beginning of WoD have seen most of the leveling content. The best you can aim for is making it the least amount of boring for most while maximizing the fun for those that enjoy it. For those that love leveling, they can opt to not use heirlooms to extend their leveling time, which coupled with zone scaling allows them to complete most of the stories on one of the contents with a little extra from the other. For those that hate leveling, heirlooms plus the speed of pre-7.3.5 leveling would allow them to blaze past it fast enough that it didn't feel terrible, and coupled with the zone scaling, they were leaving zones less frequently so they both get the story but also waste less time traveling between zones to maximize their XP gains. And if they wanted the leveling process to "teach the class", then like I suggested, up the rewards for the additional time it took. Make individual mobs harder but require fewer to complete a level.
This would, imo, achieve the most optimal solution for the "leveling isn't fun" problem, as it would maximize the "fun" as much as possible for the greatest chunk of the population as possible. They could even offer a potion that acts as an anti-leveling potion, in that it offers reduced XP (say like the XP potion from MoP, except -30% XP gains for 2 hours or whatever) that they could then buy from a vendor. Those that absolutely love leveling so much could then shift themselves into the current situation of having super long leveling experiences that they enjoy without hamstringing those that are just tired of leveling.
As far as the boost goes, there are a lot of people leveling specifically for Allied Race transmog options. That one thing is the only reason they level. I have 20+ 110s. I have at least 1 of every class, 2 of most of them, at max level. Additionally, I have been playing since Vanilla. Leveling additional characters for the transmog is the only reason to level because I have seen EVERYTHING there is to see in the old world content. I have leveled to max on both factions and finished virtually every zone (there might be a quest or two somewhere that I haven't done). And boosting kills the ability to do that.