This review contains spoilers.
Justice League feels to me, first and foremost, like a missed opportunity.
There's an interesting idea at the core of the film: "What would it mean to the world to lose Superman?" It's a clear continuation of the thematic notions that Zack Snyder has tried to introduce into his cinematic adaptations of the DC comic books.
From an alien raised as an American who has god-like powers trying to figure out his responsibilities to the world (Man of Steel) to powerful men grappling with the fact that they are no longer the most awesome figures on the planet (Batman v Superman: Dawn of Justice) to the government's reaction to this new threat (Suicide Squad) to demigods realizing humanity cannot be forced into peaceful coexistence with itself (Wonder Woman), these movies have all been in a very real way about how the appearance of Superman and those like him would fundamentally alter—or even break—the world.
And his sudden absence would undoubtedly alter it again. We get a sense of that in the film's opening moments, of a world spinning out of control because it has lost hope, been stripped of something, someone to believe in. Hate crimes, reactionary bands of terrorists, et cetera: things are bad and getting worse.
But it turns out there's something that matters more than Superman, Symbol. It's Superman, Puncher. Because, at the end of the day, the only thing Batman (Ben Affleck), Wonder Woman (Gal Gadot), the Flash (Ezra Miller), Aquaman (Jason Momoa), and Cyborg (Ray Fisher) really need Superman (Henry Cavill) to do is punch Steppenwolf (Ciaran Hinds, I guess; it's hard to tell under all the makeup and CGI)—an alien who wants to conquer the world by turning it into fire and ash because hey, why not—really, really hard.
Who are Cyborg and the Flash and Aquaman, you might be wondering? Good question! Unfortunately, we get approximately five minutes for the backstory of each: The Flash's dad is in jail, falsely convicted of killing his wife; Cyborg's scientist father put him together after an accident left him crippled; and Aquaman is … not king of Atlantis, I don't think, but annoyed at Amber Heard, who is the queen, maybe? Or, like, his adopted sister? I'll be honest, I couldn't quite figure out what was going on there. As a wise philosopher once said in a slightly different context: "It don't matter. None of this matters."
Simply put: Justice League is overstuffed and undercooked and features at least two too many new characters, especially given the remarkably sprightly sub-two-hour running time. Gun to my head, I'd toss Aquaman—even though I quite like Momoa's fratty Aquabro vibe—since, well, pretty much the entire movie takes place on dry land and he's just a weaker version of Wonder Woman in the big fights.
Miller is clearly the breakout star here; I saw the movie with a paying audience and they loved the Flash's goofy sensibility. He's the only character who really works in toto, especially given the aggressively dumbed down stakes of the team's mission. Fisher is bland to the point of invisibility as Cyborg and I don't care that it was his catchphrase on Teen Titans: whoever thought it was a good idea for his closing line to be "Booyah!" should be barred from working in Hollywood ever again.
The score, by Danny Elfman, is bland and unmemorable, only piquing one's interest when it samples from previous superhero movies. The special effects are bland and unmemorable, except for Flash's slo-mo sequences.
There are mid-credits and post-credits scenes, if you're into that sort of thing.