Im guessing that some would say the story, but lets face no MMO could ever have a good story line since the world in an MMO is always static, so no matter what the players does it is always the same (I know Eve Online is too some degree the exception here). As a player you will never feel special, you are just one more clone of a specific class, most likely forced into one specific power build.
So as a Elder Scrolls fan this is the first in the series I do NOT look forward to.