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).
The heroes we wanted all along?
All the missteps aside, there are a few bits of humor that work -- almost exclusively involving the Flash. And there is a concerted effort to try to realign these characters with the general public's expectations of them. A lot of this comes at the cost of ignoring the previous movies' events and themes, and so it's not entirely believable. But apparently, the world loves super heroes now, and it's been retconned that the world always did love superheroes.
From a more meta standpoint, the musical score includes bits from the 1989 Tim Burton Batman and the 1978 Richard Donner Superman (as well as bits from the Wonder Woman score). So despite all the efforts that BvS made to emphasize that these are "not your father's Superman and Batman", they're now trying to shove down our throats (textually, subtextually, and aesthetically) the idea that these are the same Batman and Superman that we always loved. I don't buy it.
Justice League incorporates the themes from the '78 Richard Donner Superman and '89 Tim Burton Batman.
A part of me did wonder if the inclusion of the '89 Batman theme (which was actually pretty prominent in Justice League's score) is maybe meant to imply that Batman and Batman: Returns are now meant to be considered as possibly having taken place within this movie's continuity. It seems like a stretch, especially considering that the '78 Superman is certainly not intended to be part of this canon. But it is possible that Ben Affleck is maybe supposed to be a 20-years-older Michael Keaton.
You can't miss the shot that you don't take...
Justice League benefits tremendously from the abysmal failure of previous DC movies. It's not unwatchably bad, but it is pretty dumb. Much like Wonder Woman, the mere fact DC put together a passably watchable movie seems like a miracle, but let there be no second-guessing that Wonder Woman is certainly a much better movie. The dumb plot and complete pivot in tone and style betrays Justice League's reactionary nature, and it suffers from its attempts to distance itself from Dawn of Justice while simultaneously having to tiptoe around (and retcon) that movie's asinine plot details.
Yeah sure, BvS was a stupid mess, but hidden underneath that stupid mess were some genuinely good concepts and thought-provoking ideas, some of which were even better executed than Marvel's own Civil War movie (which, admittedly, got much better for me after a second viewing). Abandoning those sorts of ideas in favor of a strict "the heroes are always right, and they always win" mentality is disappointing. Just because I want the heroes to actual behave like heroes doesn't mean that I don't want them to have to struggle with their place in society. Not only does Justice League discard those concepts, but it also tries to retcon them out of the previous movie by pretending that everyone already thought Superman was the greatest thing since sliced bread, and that the world had gone to shit after he died (even though that's completely antithetical to everything that both BvS and Man of Steel established). In the process, Justice League fails to really feel like it's even trying to be about anything. It's a completely disposable, insubstantive, punch fest, but with way too much standing around talking about nothing.
Oh, and the movie is so unsure about the franchise's future, that they basically pulled out any potential build-ups for Darkseid, New Gods, or any of Batman's nightmare premonition from BvS in favor of a post-credits tease of Jessie Eisenberg's Lex Luthor setting up a Legion of Doom completely out of left field.
All the sequel bait from the BvS is dropped in favor of a setup for Lex Luthor's Legion of Doom.
Perhaps the biggest problem for the future is that now DC movies will probably start to look and feel more and more like Marvel movies, but without the well-thought-out world-building and substantive characters. I'm already feeling burnt out on Marvel movies (and comic movies in general), so the idea of having to sit through two or three Marvel films (and a terrible Star Wars spin-off) per year plus another one or two Marvel-ish DC movies really does not sit well with me. Perhaps that's DC's fall-back anti-Marvel plan: kill the market for comic book movies by diluting it with mediocre, homogenous films until nobody wants to see the Marvel films anymore either. Hey, it worked for Activision with regard to killing the success of Harmonix's Rock Band games!