D2 , D3 (act 1 only, other acts are not so dark) dark... like how Lordaeron/Duskwood/Theramore was like during vanilla. (closest thing to dark) death and detailed corpses everywhere, no jokes no gimmicks... just a sense of utter vulnerability and lingering danger, little unsolved mysteries at every corner.
someone mentioned dynamic events at timeless isle and not many quests. that's a good model for WoW. we should see little dynamic events in zones. (like an attack on Aerie Peak by the forsaken, or a caravan under attack at blasted lands.)
combine that with a dark theme and we can see some interesting things in WoW.