So hard to choose what not to keep... among the victims being Mythic Keystones, World Quests, bad lore, short patch cycle... If forced to choose I suppose It has to be
Kill World Quests - feels like a lazier, more efortless version of any normal quest blizzard makes to then make Into a constantly repeatable quest, making every quest you do never have meaning, the rewards are meager, enough to keep you going like you're gambling but not really, although the feeling of wanting more Is there despite the small amount, the reward Itself Is another disasteour entirely. Daily Quests still feel superior and with the emissary system could be really good.
And
Keep Emissary system - with some tweaking It can work wonderfully with daily quests.