4. The Quips Need To Suit The Actors
It's widely accepted that Whedon will be punching up the character work and adding humour to the DCEU, but after writing for the Marvel Cinematic Universe for so long, he really needs to know both these new characters and the actors playing them.
Considering that Whedon had been helping out on the production for a while before Snyder departed, he's clearly had time to look at dailies and get to know these versions of the characters, so hopefully this will ensure that he's giving the actors and characters material that suits their sensibilities and doesn't play as forced.
After all, many movies that go through re-writes and contributions from multiple creative voices end up feeling schizophrenic, but if Whedon's truly going to lend a helping hand to the production, he'll need to be in perfect concert with a cast who are of course much more familiar with Snyder's way of working.