Seriously, leave now and check back after the movie!
Alright, moving on.
As you know, the cosmic powerhouse called Thanos makes his big screen debut at the end of Joss Whedon's awesome flick. An alien informs the Mad Titan that Loki has failed to conquer Earth, and to face the newly assembled superhero team would be a death wish. Thanos turns his face and delivers a purple smirk that gave every fanboy goosebumps. Why does he smile at the challenge? I have two theories.
source: my blog