One of the things that WoW uses in it's simplest form is the "hardcore quests" (or high-level quests). Adventures targeted to high-level characters, usually requiring special equipment (which means raiding again and again dungeons to get...
I've just found in Slashdot an entry called "What Are Some of Your Favourite RPG Quests?" and I'm thinking what is my favourite quest... I think that the best one I've played was the "alternate world" in UO (in Berks Worlds...