For me its probably the stories and/or atmosphere.
First of all, implementing voice acting for all npc’s and quests has made the game so much more enjoyable and immersive than other mmo’s. Secondly, Im a big fan of fantasy and ESO just has almost everything I could wish for in terms of lore, environments, etc.
I also just finished the aldmeri dominion questline and I got to say its my favorite one so far. Murkmire, and orsinium come close second. Most of the quests I have played have been very entertaining and I can’t wait to play elsweyr.