Let's start with the most obvious weakpoint: the villain. Steppenwolf is a laughably bad villain, ripped straight out of a 90ies direct-to-DVD film. He is just a big bland CGI character with absolutely no personality whatsoever. He has no motivation other than being the generic evil guy trying to destroy the world. Apparently it's not even hís plan. If I understood correctly, he's the henchman for some other character that we never learn anything about. His entire design was just uninspired, and he looked and sounded more like a weaker version of the Beast from the live action Beauty and the Beast than one of the strongest characters in the world, hellbend on creating destruction.
Then there are the problems with the entire way that DC is approaching these movies. Making this an ensemble movie, just like with Batman v Superman, feels entirely unearned and a cynical attempt to cash in on the widespread brand recognition of Batman and Superman, and to a slightly lesser degree Wonder Woman, and of course the success of Marvel. It's really hard to look at this film and not conclude that this is DC's failed attempt at creating their version of The Avengers.
However, whereas The Avengers' meetup felt natural, having been build up throughout its previous 5 movies, Justice League decides to introduce half of the entire league in just this movie. This makes most of the first half of the movie an uneven and boring bunch of random character introductions. Only Wonder Woman and the segments on Themyscira felt natural, because we were already introduced to their culture before and therefore had some connection to them.
The rest of the Justice League itself is also lackluster. Jason Momoa's Aquaman was kinda interesting, but it once again feels incredibly cynical that they would introduce the character in this movie instead of in his own one, just because they knew that it would be hard to get people excited for an Aquaman standalone movie without an introduction to the character first. They don't care that it's to the detriment of the quality of this movie, it's just a way too get people to buy into a future film, exactly as they did with Wonder Woman. In this case they actually would have done well to look at Marvel, as they were able to introduce obscure and silly looking/sounding characters like Ant-Man and the Guardians of the Galaxy in standalone movies, and make it a critical and commercial success.
Cyborg was just boring and uninteresting. He has no personality, there's nothing to his character, because there was not enough time for a proper introduction. Even his suit is making the decisions for him sometimes. The CGI was also quite distracting, as his face looks rather fake.
And of course while The Flash has been around for a long time, I find it hard to believe that they would have introduced his character the way they did without the success of Spider-Man in 2016's Civil War. He's pretty much an exact copy of Spider-Man, playing the teenage boy that is overenthuastic about being a superhero, and deriving humor from that.
They even found a way to make Batman look uncool. He's pretty much just collecting the team for most of the movie, and does very little on his own. What is most disappointing is that Ben Affleck is never really Bruce Wayne. Whenever you see him dressed normally on screen, it's just him doing Batman stuff without his suit on. Even Wonder Woman has a scene of her being Diana Prince, but it's all only Batman throughout.
Then there's Superman, whose return was very poorly done. First of all, if you have a character that is supposedly dead, don't make the actor's name the second one to pop up on the opening credits. That's a surefire way to ruin any potential surprise about his return. Of course that return was inevitable and obvious, but at least put in a little bit of effort to hide that fact. I don't even have to put in spoiler tags in here because DC doesn't even consider his return a spoiler.
Secondly, because his return was so quickly - halfway through the chronological follow up to the movie in which he died - there is absolutely no relief or emotional climax to his arc, or any sense of urgency, really. You never really care that Superman's death has supposedly thrown the world in disarray, because his death barely has had any time to have resonated with the viewers. Hell, his supposed death didn't even make it through the end of the previous movie without some hint of his upcoming return.
If they had some patience and balls, they would have had him stay dead for a few films, preferably those in which the rest of the Justice League got introduced, and in which the threats get increasingly challenging and hard to overcome due to Superman's absence. This way when he finally does return after the rest of the League is facing a truly frightening and unstoppable enemy, his come-back would feel earned.
Overall, there's so much wrong with this movie that although I wrote so much, there are still many flaws left untouched (like Zach Snyder being Zach Snyder doing too many Zach Snyder things, or the nonsensical process of resurrecting Superman that actually put them in more danger than before). It's amazing that after the disaster that was Batman v Superman, they went pretty much the exact same route for its follow-up, setting up an illogical and unearned conflict and resolving it in a cliched and nonsensical manner.
At least Batman v Superman had some really cool action scenes instead of the uninteresting CGI fest that was Justice League. And while Jesse Eisenberg's Lex Luthor was a terrible villain, I'd take him over the soulless CGI monstrosity that is Steppenwolf any day of the week. Justice League is a terrible movie in every regard, and really does not bode well for the future of DC's cinematic universe.