Given that two other directors have left the project over "creative" issues, we would have given "a tolerance for interference" as the answer. But depending on how you count it, the 2018 Flash movie is on its third or fourth director.
This leaves poor Ezra Miller writing a list of qualities on a piece of paper and hoping his dad tears it up and puts it in a chimney so that the perfect director will descend out of the sky, Mary Poppins-style. Here's what Miller told Cinema Blend:
I think that the most important quality is the open heart. I think that The Flash has to be a story that is heartfelt. Because I think that's where the lightning really is.
The lightning was inside of us all along! Poor Miller, he seems so earnest but this is beginning to look bad.
Originally, the Flash script was going to come from Phil Lord and Christopher Miller, the duo behind the LEGO Movie. Presumably, Warner Bros. was hoping they'd direct, but they were too busy. Then Seth Grahame-Smith (Abraham Lincoln: Vampire Hunter) was hired to write and make his directorial debut. He left for the classic reason of "creative differences". Next, Warner Bros. hired Rick Famuyiwa (Dope) to helm the project. He left last week because they couldn't "come together creatively".
Faced with finding another director and getting production done in time to accommodate both the already-announced 16 March 2018 release date and Miller having a commitment in July, Warner Bros. is in trouble. Anyone they hire is going to know they were a fourth choice. And a rushed schedule is rarely a sign of success. At the very least The Flash is going to have to be delayed while WB finds a director with an open heart.