5. The Role Of Editor David Brenner
First off, no, Justice League is not 170 minutes long as those recent rumours claimed. However, it is certainly going to require a steady editing hand to bring such an ambitious project and messy, sprawling production under control.
The film is being edited by David Brenner, who also edited Batman v Superman and Man of Steel, and though the former film in particular was incredibly sloppily edited, it's important to remember how extensively Warner Bros. meddled in the editing process.
Simply, Justice League's quality will be heavily determined by the working relationship between Brenner and Whedon: if Whedon can help Brenner iron out the pacing issues of his prior Snyder collaborations, then he'll probably manage to bring the best out of the editor.