I legit cracked up when that cheerful zany music was playing during the scene showing Superman's funeral. I wasn't sure if that was supposed to be a subversive commentary or if the music editors really messed up but I just couldn't help myself. It was just so jarring to hear what should've been a zany hero entrance song playing while you've got bagpipes and a coffin being paraded through Metropolis.
El Diablo was seriously the best part of this movie, which I wasn't expecting because I didn't know who he was prior to watching. His character arc was pretty solid, and I really liked the transition from volatile gangster to pacifist to self-sacrifice, though it wasn't nearly as good as it could've been. If it had been set up better (like with more and lengthier team building to make that 'you're my family now' comment actually make sense), that bit where he goes all twelve-foot tall flaming skeleton could've been DC's "I'm always angry" moment.
There was way too much time spent on Deadshot and Harley's backstories at the beginning. Everything shown there was worked in way better later on in the film. It would've been way better to just have Joker bust in and break out Harley from nowhere instead of trying to shoe-horn him into the rest of the movie. It's not like everybody doesn't already know who she is. Same with Deadshot, would've been way more impactful to have him be a jokey tough guy for the whole movie and then shock everyone by asking for time to go help his daughter with her math homework at the end. Then maybe they would have some time to add in a team-building warm up mission and maybe a montage to justify the team actually becoming a team by the end. As it was, nothing felt earned.
And Katana had absolutely nothing to do in this movie. Even Killer Croc had his "No, I'm beautiful!" moment. With Katana, it felt like there was a huge subplot that just vanished. Rick Flagg mentions twice that her sword sucks out people's souls... and then we never get to see the sword suck out anyone's soul. It begs the question of why did they even mention it. It would have sufficed to just say she was sword lady and leave it at that.