Posthumous Credit: Miguel Ferrer, who voiced Deathstroke, died on January 19, 2017, roughly 3 months before the movie came out. Coincidentally, his role in the movie was announced on the day of his death.
Considering that the movie was in development in 2006 before being cancelled a few years later and then brought back to development in 2016, who knows how different the movie might have been if it had never been cancelled in the first place. However, it was always intended to follow the comic rather than the Terra arc as seen in the animated series.
James Tucker mentioned that there was originally a post credits scene of Blackfire scheming to go after Starfire that would've set up a third movie.
Writer Conflicts with Canon: James Tucker's claim that this movie's Kid Flash is Barry Allen rather than Wally West doesn't mesh with what the other movies in the DCAMU canon establish about Barry's history, most jarringly the fact that Kid Flash is on the same team as Robin when Barry Allen was shown to initially believe Batman wasn't real in Justice League: War. Not only that, but in The Death of Superman, Barry remarks maybe he should join the Titans and wonder if there's an age limit. A comment that wouldn't make sense if he was part of the team before hand.
mentioned that there was originally a post credits scene of Blackfire scheming to go after Starfire that would've set up a third movie.