Batman V Superman: 10 Mistakes It Made That Justice League Must Avoid
2. Giving Zack Snyder Too Much Freedom
Zack Snyder has a very distinctive directorial style, but very rarely has that style resulted in a movie that's loved across the board. Generally speaking, his movies lean more toward style than substance, usually lacking a compelling plot to complement the stunning visuals. If online rumors and speculation are to be believed, he and his team had complete control over Batman v Superman, and that's a mistake - he needs some help in the story department.
Snyder didn't actually script the film - that was left to Oscar-winner Chris Terrio - but given the hodgepodge nature of the finished product, it's clear that Snyder gave Terrio a ton of notes that Terrio had to try and work in. It didn't pan out.
Snyder isn't a great writer, but with the amount of control Warner Bros gave him he was allowed too much influence over the movie's script.
So, for Justice League, Warner Bros should take back control. Leave the writing to the writers (the Oscar-winning writers) and let Snyder focus on what he does best - making a movie look great. Too many cooks spoils the broth, and if every single one of the filmmakers is focused purely on their job alone, perhaps each of those jobs will be done to a higher standard.