So, I'm going on record as all you Justice League haters are overly critical.
TL;DR: Justice League is totally fine. It's not Avengers quality, but it's honestly fine. It's better than Avengers: Age of Ultron. Given the turd that WW84 was, I think if we all look at JL again we can see it's a completely fine movie.
Justice League by itself is a difficult movie to pull off: we have to cover the basics of a handful of super heroes, including Cyborg, Flash, and Aquaman. We have to give enough background to make them interesting, but not enough to bore the audience. It's hard enough to introduce multiple characters, and then give us a bad guy that we care about, and then a plot. That's a lot for one movie to carry. ... Not to mention that the director's daughter committed suicide during post production, which led to them bringing in Joss Whedon to re-edit/finish the movie.
Plot: The plots is fine enough. It's established from the beginning. Batman discovers there are these three boxes, and these parademons, and knows he is dealing with something bigger than him and extra-terrestrial. He immediately decides to "assemble the team", and goes after the other members of the Justice League in an attempt to recruit them. Literally within five minutes of the movie starting, we have a defined plot, and know who the antagonist is. So we have a mission we have to accomplish. There isn't much, if any, meandering, and the plot never loses its course. It's always "recruit good guys, find boxes, fight bad guy." I actually liked the McGuffins where it is three boxes of power given to the three different species (humans, Amazonians, Atlantians)... boy, sure sounds like Lord of the Rings (humans, dwarves, elves.)
Batman: Batfleck was fucking fine. Yes, he used guns. He's not fighting the Penguin or any other human being. He's fighting paranormal demons from another world along with a bad guy that is four times his size and is a conquerer of worlds. He knows he is outclassed and is doing what he can to save the world he lives in. He reluctantly takes the role of leader, while wishing others (Wonder Woman, Superman) had the role. He's fine as Bruce Wayne and Batman. Yes he looks "heavy", but part of that is the style of his armor.
Flash: Flash was fine. Ezra Miller was funny and insecure in the role. Again, this was some nerdy kid who is into Japanese cartoons that gets his powers. He isn't some badass trained fighter. He's scared, he doesn't know what to do. He just has a skill that Batman is interested in and is trying his best to help. I truly enjoy the scene where the League encounters Steppenwolfe for the first time, and Batman coaches Flash with "Get in, get out, don't fight, just save one." And Flash learns from it. Flash also has some great slow motion scenes, for instance when Wonder Woman is fighting Steppenwolfe and drops her sword, and Flash runs down and pushes the sword back up towards her. I wish they didn't end that scene with him tripping, because it's ok for him to excel without comedic relief. He was completely fine in the entirety of the movie, with no complaints. Imagine you are Flash, and all of a sudden your thrust into this world where, at one point, you're taking on a newly resurrected Superman, and fucking Superman is taking a punch at you. How would you act?
Cyborg: His introduction / first half of the movie is bad. I wonder how much of this was the result of two directors? His initial scenes, acting, writing were really bad. But in the third act he's totally fine, he's funny, and enjoyable. However, I thought it was great writing for how he was tied into the story. He's born of a motherbox, and his dad is a scientist that works with the boxes. He gets dragged into the fight primarily because his father is kidnapped by Steppenwolfe who is looking for the boxes. It tied together nicely.
Aquaman: This is an alpha male, badass motherfucker. Momoa was fantastic. He looks insanely awesome, and he's such a "bro" that every line that comes out of his mouth is fun. "My man!" "All right!" I cracked up each line. And his scene where he's sitting on the Lasso of Truth was hilarious. His action sequences are fine, all around fantastic. The scenes in Atlantis when Steppenwolfe is stealing the stone, those were poorly done, and I'm glad they figured out underwater action / dialogue for the Aquaman movie.
Wonder Woman: Her opening scene is fantastic, and she's honestly great throughout this movie. She continues her role as a badass Amazonian warrior. She stands toe to toe with gods and conquerers without a second thought. She's fearless. She's beautiful. There's something about Gal Gadot where she is wholesome, lovely, and just so intoxicating that every scene with her on the screen is a treat. Her action scenes are fantastic and she looks great doing them. Her weak parts are her constantly lamenting for Steve Trevor. We get it, you miss him, it was 100 years ago, shut the fuck up.
Superman: He was totally fine. Yes his lip looked weird, are we really going to trash a movie based on that? It does suck that he's so overly powered, but he is the biggest badass in comic books, so what can we do? It was interesting when he was resurrected and was confused and fought his friends. I thought his acting was off then. I'm interested to see what happens in the Snyder cut with him, because he was barely in this movie.
Steppenwolfe: Terrible villain. One of the worst. As bad as Ultron was. He was uninteresting, looked like CGI from 15 years ago. By far the worst part of the movie.
In summary, this movie had its faults, with it being a mish-mash of two directors visions, some poor writing, a bad villain, but overall it was fine, fun, coherent, and linear. But it's absolutely not the giant turd that people make it out to be. Especially when compared to WW84. Please give this a re-watch!