They can use common sense. No one likes to do the same quest a million times. So for each character you raise to 90, you get a permanent leveling buff for your other characters. But that very first 90 should still be an epic journey- by the time you get there, you should have learned your class in and out by necessity because you had to work your way through different difficult situations, not because the devs pushed off the burden of teaching you how to play well to fucking websites that don't even belong to them. Hell, it was harder to hit 40 in vanilla than it is to reach max level now. AND the content on the way is stupid easy. No one has to group, no one has to work together for quests. It's a welcome mat for anti-social assholes.
Edit: I've said it before, and I'll say it again. Leveling design (and some game design in general) on this game has gotten really, really lazy. You can tell when a business shifts its agenda from "making a good product and thereby a good profit" to "making a good profit however we can". I'm fairly certain the focus isn't creating a good game any longer, it's simply keeping as many subscriptions for as long as possible. That approach never works in creative endeavors, the end product ends up stale, two-dimensional and lacking. You can see it with TV shows that took off and were too successful for their own good. Surprise hit movies that get shitty soul-less sequels pumped out. And it happens to games. Sure, they can fool people for a little while, but after people realize they're not buying "Cool Thing 2.0", but "Supposedly Cool Thing designed to make you buy it again and made as cheaply as possible to get as much money out of it before people wise up and realize we're not even really the same fuckers who made the first product", they STOP BUYING IT.