The biggest challenge when leveling

How many characters do my reads have? 1? 10? the full 50 available? Every person makes they're own amount, with their own tastes. The great thing about leveling is choosing a "subpar" non-raiding, non-pvp spec that suits all your own tastes.

I play with my now ex-girlfriend on occasion. We have a set of leveling characters that we will NOT play unless the other is there, effectively dinging within moments of each other. She's a shadow preist, and I'm a combat rogue. Because we generally level together, mobs die far quicker than on our own, so we very much build our specs around each other. She's taking aggro reducing talents as mindblast followed by holy fire tends to send things sprinting in her direction, and I'm taking talents that increase any survivablity I have, parry, dodge, what have you.

But what if you're leveling by yourself. Do you want to know whats the hardest part about leveling?

It's levels 40-50.

Didja hear that? Now I'm sure many people will disagree with my about this, but I hold firm in this. I've got around 40 characters, most aren't very high leveled and the few that are decent are floating around this level range. They werent all made the same time, so its not a matter of burning myself out on a specific character, or leveling area.
There's a few reasons and I think I've gotton most of them down pretty well. First off the hubs that start around level 40 (feralas, tanaris) are in the ass end of nowhere. I don't know how alliance doesn't want to kill themselves trying to get there, because I sure do dread the trip, and I tend to hang out in thunder bluff more than any sane man should when I'm a hordie. The second reason I can think of is most classes stop any notable developement for a while. 40 is a HUGE level for most classes, but the levels following them become "play with your toy" levels while they come up for new things to give you. Prot warriors learn shield slam, the corner stone of their tanking, enhancement shaman learn to dual wield, boomkins become just that, etc etc. It's a great level but man will you be bored after that. Whats worse is you don't get much of a consolation if you hit 50. You get some of in my opinion the worst questing hubs, and a minor glyph slot. There's not much to work with, and it's unfortunate.

I hope the mastery system becomes a prominent change to these problems, I want intersting skills AS I level, not just at mile stones, as a carrot on a stick to guide my leveling.