And no, it's not because of fans reception to Age of Ultron
Joss Whedon says he's done putting his "fairy dust" on the Marvel movies, but it's not because of a falling out, it's because he just wouldn't be able to get anything else done if he stayed. During a Q&A at Oxford University last month, Whedon revealed that he was no longer consulting on Marvel movies despite earlier reports that that's exactly what he'd do through Marvel's upcoming Phase 3 set of films.
"I was their sort of consigliere for a while ... and I sort of had my finger in all of the films in the second phase," Whedon said. "Then I had to concentrate on only Ultron, and know that when it was done I was just going to stop.
Source
By the sound of it he wants to try new things, which I'm all for. Best of luck to him.
Discuss.