I'm speaking from personal experience. I've leveled quite a few toons on several different realms.
What makes leveling boring is when it becomes bland. Quests are all too similar with a few stand out ones. By the time you hit max level, you've basically done the same quest 1,000 times. "Go in the cave there and kill 25 baddies and get that orb from the big baddy." You don't even begin to appreciate the designs because you've seen it in another zone, using the same cave design in a previous expansion and the same old spell.
A challenge to leveling would change things up, for sure.... BUT making classes utilize nearly their entire repertoire... by the 5,000th kill it gets tiring and old AF.
Leveling shouldn't be long but it should feel satisfactory.
Make questing different, use brain puzzles, etc etc... someone has to figure it out.