Whedon clarifies the whole Thanos/Avengers 2 thing; sort of

Thanos appearing for no apparent reason at the end of "The Avengers"

After implying at the end of "The Avengers" that we'd be seeing Thanos as the bad guy in "Avengers 2" Marvel threw a wrench into the expectations of Thanos fans last month by announcing that the sequel would be titled: "The Avengers: Age of Ultron". Ultron? I don't remember seeing him in any teaser footage? What happened to Thanos?

Well, Whedon has apparently heard some of the kvetching his sequel title generated because in an interview with TotalFilm he addresses it: “We have to stay grounded. It’s part of what makes the Marvel universe click – their relationship to the real world. It’s science-fiction, and Thanos is not out of the mix, but Thanos was never meant to be the next villain.”

It's your movie and you can do what you want, but don't blame fans for the expectations you set in motion Joss.