What I am really looking forward to in Legion is the development of the classes to make them more unique, distinguished and special. I like that Blizzard is making it easier to immerse yourself in your character by making you feel different from the other classes, like the class halls, legendary artifacts and questlines. It has been done before in WoW, with a few quest lines (like the warlock green fire quest chain), but never seen much impact in how you might see your character.
My absolute favorite WoW moment was in WotLK, when doing the quest chain for the Wrath Gate in Dragonblight for