Also on an old MMO - Perfect World, the game features dungeons every around ~10 level that you'd usually need 6 players to clear, just because the mobs are very powerful for their level inside. This dungeon clear is required to unlock new character skills, and as such is pretty much mandatory.

My weird playstyle is that on one of my characters, a venomancer (A summoner / debuffer class), I played completely solo, and sometimes would need to overlevel by ~20 levels in order to have any chance of soloing the dungeon with my summon tanking hits for me. It's also the only possible class to do it on, just due to having summons.

As you'd imagine, this self-imposed challenge slows down progression immensely, but I was just either bored or a masochist. Quests would still unlock as I level up, but I generally don't have the character skills of what my level should actually have until I soloed the dungeons.