Don't know how to post links, but a short article on Playstationlifestyle just a few hours ago details why the ESRB chose to give ESO an M rating.
TL;DR:
* Some sequences depict large amounts of blood streaming up-close as vampires attack/feed on characters.
* Some environments depict corpse piles or skeletons hanging from torture devices
* Text descriptions or dialogue sometimes contain references to sixual material and/or innuendos (ie Lusty Argonian Maid, Molag Bal's vampire story)
And finally some things about alcohol consumption:
One sequence prompts players to engage in a drinking contest, resulting in the central characters blurred vision and slurred speech.