Each world has one big instanced area that's really hard and takes a long time to complete. Those areas are optional, meaning you don't need to complete them in order to advance to other worlds and complete the storyline quests. In Wizard City, it's the Sunken City quest, in Krokotopia it's the quest in the well of souls that the mander gives you, and in Marleybone it's the Kensington quest. I don't know if there's one in Mooshu. Does anyone know if Mooshu has one of these quests?