I think Deathwing's representation was well done during Cataclysm, especially him roaming the continents etc. The last real big massive villain we had imho. And Arthas was quite present throughout WotLK. And with the Scourge being around since Vanilla he was always somehow present in game. Naga were around since Vanilla too but Azshara never was featured or anything like that. She needed a real build up but Blizzard's writing failed in this regard.