I guess Joss was already on board to do a Batgirl movie but it is interesting to see the writer/director of both Avengers movies taking over on Justice League. I'm kind of surprised Marvel doesn't put something in their contracts to prevent the main people from their cinematic universe from going over to DC. I guess it's not any stranger than J.J. Abrams doing the modern Star Trek and Star Wars movies though.
Anyways, it's terrible news for Zack Snyder and his family but if anyone has proven that they know how to handle these kinds of movies it's Joss Whedon. I just wonder how much input he will actually have. He might just be supervising the plan Snyder already laid out.
Edit: Since the movie is coming out this November Whedon obviously will have very little input. I didn't realize we already so close.