For the past 4 years of so, Marvel has been slowly building the universe finally unveiled in “The Avengers.” One way to do that was by the inclusion of Easter Eggs in other superhero movies, so it's no wonder it did the same with the aforementioned film.
As we also noted in our review of “The Avengers,”
this is one of those rare instances when you shouldn't leave the theater as the closing credits come because Marvel included an Easter Egg at the end.
It has now released that scene as a standalone teaser, meant to drum up publicity for the DVD / Blu-Ray release: as it should, since it literally reveals the sequel's main villain.
He is Thanos, an Eternal who is pretty much in love with death, which explains his grin when he is told that fighting humanity equals “courting death.”
Since chances are that few of the many millions who saw “The Avengers” had any idea about who Thanos is, this makes this Easter Egg one of the nerdiest ever included in a Marvel film.