From the start, Justice League seemed to be slipping off the rails due to its awkward editing in the first act. It certainly doesn't help that the opening scene is terrible, uncanny valley, cell phone footage of Superman. Who in their right mind thought that would be a good opening scene?
So this movie takes place years after the events of Dawn of Justice, but Batman and Wonder Woman are just now getting around to recruiting the others? Are we to believe that Flash, Aquaman, and Cyborg had evaded their efforts for so long, only to suddenly all pop up at the same time? Then we cut to Steppenwolf suddenly appearing in Themyscira to steal his McGuffin box and butcher a bunch of Amazon. It's a scene that feels like it should maybe have been earlier in the movie in order to act as a catalyst for Batman to stop pussyfooting around and just go get Flash and Aquaman, or should have been after the bulk of the team has joined up.
As soon as Cyborg showed up, I felt the movie falling completely off the rails. Everything was getting too convenient -- too easy. There just wasn't any real resistance for the heroes. The villain completely failed to create any real stakes or sense of threat, and just has no real presence in the movie at all. Even when the characters are saying that "we're all probably going to die", it doesn't feel like a low point for them. By the end, the heroes are gleefully punching the bad guy, and Superman just shows up a toys with him. Nobody seems to be taking anything seriously. There's no gravitas. Even the Marvel movies, as colorful and upbeat as they tend to be, always have moments with gravity. Justice League tries to go the route of Avengers by pretending that the heroes don't really get along (until they have to), but it never feels genuine.
So now DC has completely squandered both the death and resurrection of Superman. These are two watershed moments in comic book history that are never going to have the same impact for movie goers. And hey, let's leave the magic, world-destroying McGuffin unattended while we wrestle with Superman, so that the bad guy can literally just beam down, pick it up, and beam away.
Why is the team just now coming together?
What the heck were Batman and Wonder Woman doing in the years since BvS?
In addition to poor editing and poor plotting, the movie is also just plain ugly to look at. I want to praise the movie for being brighter, more colorful, and not as washed-out as Man of Steel or BvS, but I can't because the movie just looks terrible. Superman, Aquaman, and Batman's costumes look particularly phony in the brightly-lit environments of this movie. Steppenwolf and Cyborg stand out as blatant CGI monstrosities. And Flash and Wonder Woman's costumes aren't far behind on the ugly scale. Flash's costume looks like it is being held together with pieces of metal wire and tape and looks like a cheap cosplay outfit, even though Bruce Wayne gushes over how technologically advanced it looks. What, Bruce, you couldn't give him a kick ass, actual technologically advanced costume like what Tony Stark did for Spider-Man in Homecoming? Half the time, Wonder Woman's costume suddenly looks too big in the top (like it's about to fall off of her) and too small in the bottom (with her cheeks hanging out from under her skirt). Jason Mamoa and Henry Cavill get topless for whole scenes, so the ladies get plenty of beefcake to ogle. But Wonder Woman is the only one with her ass hanging out of her actual costume.
In general, the color-correction looks awful. You can view early trailers of the movie, and then newer trailers and see that entire scenes were changed from being general night-time footage to being in broad daylight or having some weird red filter applied to them. I get that it's a reaction to the general consensus that the other movies are too dark (visually), but as movie-makers, someone had to have known that the costumes are designed to be shot in certain lighting conditions, and arbitrarily changing those conditions completely changes the way that the costumes look.
Even the trailers went out of their way to highlight gratuitous Wonder Woman upskirt.
There's also a few moments in which the heroes seem to completely disregard the preservation of their secret identities. Batman calls up Alfred within earshot of a thug, and Louis stands in the middle of an open park, in broad daylight, yelling Clark's name with policemen standing around her (and presumably a large crowd watching from a distance)...
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