The post credit sequence reveals the shadowy figure in space that talked to Loki at the beginning of the movie, as he's talking to an unseen figure and talking about how they had underestimated the humans and their capabilities.
"To challenge them is to court Death," he says.
CHILLS ran down my spine when I heard that line as surely every hardcore Marvel fan knew what which character that specific reference alluded to, and sure enough the unseen figure turns to the side and reveals himself as Thanos as he grins and the scene cuts to black.
The reason I'm particularly excited for this is because Thanos is absolutely my favorite character in the Marvel universe. In a nutshell, he's one of the deadliest villains, having both physical prowess as well as being incredibly brilliant. In the beginning, he was more of a one-track villain that wanted the world/universe to himself, but has since grown into an incredibly complex character.
Jim Starlin, the writer who created Thanos, had originally created him as sort of a Marvel version of DC's Darkseid, and you'll notice a lot of parallels between the two if you're familiar with the latter. Anyhow, if you're not familiar with Thanos and arguably the most famous story arc he was in, The Infinity Gauntlet,
this article sums things up rather nicely.
But holy cow, I still can't believe Marvel is going this route; it's amazing. If they're setting up Thanos as the villain in the next Avengers, we'll likely get a few cameos of him or key moments in post-credit sequences that will continue to set the stage for the Avengers 2. One thing in particular you'll notice was that the Infinity Gauntlet was in Odin's treasury as shown in Thor.
It's likely that Thor 2 may include Thanos in some fashion (whether it be actually in the movie or post-credits) in order to steal the gauntlet, further setting up the story. Several theories also suggest that he might even fight Thor, only to have Thanos defeat him to show how unstoppable he is, and in the process set up the beginning of The Avengers 2.
OH AND ALSO-
It's almost a sure thing that Damion Poitier has been cast as Thanos, as his tweets have somewhat hinted at, as well as the fact that he's credited as playing "Man #1" (on IMDB), which definitely
is not an extra (considering his pedigree). Also, his facial profile seems to match Thanos as well; how about that.
