I've seen a few reviews that claim you don't get enough backstory for the newer team members, Aquaman, Flash, and Cyborg, but honestly, knowing more about their history isn't essential.
You get a glimpse into each one's world to give you a hint of who they are, and that's really all you need. If the movie went any deeper you'd be expecting their story arcs to play out during this film.
Director, Zack Snyder, later replaced by Joss Wheden, due to a family tragedy, always said JL would be a lighter, more hopeful film than BvS. That's what JL is. Whether that can be attributed to Zack or Joss - who cares?
What matters is what ended up on screen.
Bruce Wayne/Batman (Ben Affleck) is a lot less angry but still very effective alongside his more powerful allies. He still doesn't really get to be the world's greatest detective in any obvious way, but he does anticipate what's coming and is the driving force behind finding forming the team. Plus you get to see one or two new toys.
Diana Prince/Wonder Woman (Gal Godot) is consistent with what we saw in BvS and her solo movie. Here she gets to be the seasoned veteran hero, who is no longer bemused by the world outside her home island. As such she gets to continue the great working relationship she has with Bruce/Batman that was a highlight for me in BvS. I just hope future movies never turn it into a romance without good reason.
Arthur Curry/Aquaman (Jason Mamoa) seemed a lot angrier and more threatening in the early trailers but lightened up a bit in later ones, does the job he was hired to do - make Aquaman cool again. I wouldn't say he got me as excited for his upcoming solo movie as Gal did for Wonder Woman's solo movie however, I will be more interested in seeing it when it comes around.
Barry Allen/The Flash (Ezra Miller) is a younger, more awkward version of the character that may be a little jarring if you're a fan of the CW's Flash TV series. Personally I find Ezra's version of the character more interesting to watch (and I have seen the entire first season of the CW series - which I bought). Ezra kind of plays it similar to Tom Holland's Peter Parker/Spiderman in Spiderman Homecoming, with the wide eyed awe of someone geeking out over meeting his heroes.
Victor Stone/Cyborg (Ray Fisher) for me, is the character I was least familiar with, only ever having heard of him in relation to this film - and I think the character is included in the Teen Titans, children's animated series that I've caught the occasional episode of. Other than, he seemed to be very confident using his powers for someone discovering new powers every day, he held his own as a very watchable team member.
Clarke Kent/Superman (Henry Cavill). It's not really a spoiler talking about Superman. Since the end of BvS you knew he was coming back from the dead - if he was even dead? The highlight of this movie is finally getting to see Henry play a version of Superman that isn't an emo, why do I even bother, whining mess of a character. He finally gets to be Superman proper, and even have a little fun with it.
The big bad guy is Steppen Wolf, some kind of alien god-like creature. His plan is to unite the mother boxes (that we got hints of in BvS) and destroy/take over the Earth. Some critics have bashed this movie for such a simplistic villain but he's pretty much on par with... almost every big bad Marvel villain in the MCU. Not a single one of them want to take over the Earth for any reason other than more power.
Kind of makes you miss the days when Lex Luthor was hatching real-estate schemes don't it?
My point is, a cookie cutter bad guy is not a reason to stay home on this movie. He's not the reason you go to or stay away from a JL movie.
You go to see some of your favorite heroes be your favorite heroes. Although the arc of this movie is fairly straight forward, along the way you get to see some great individual fight scenes with Batman and Wonder Woman, along with various character pairings and earlier partial team fights before the climactic battle.
My only disappointment with the film is all the secondary characters are pretty much reduced to cameos. J. K. Simmons, for example, as Commissioner Gordon. Only Bruce's Butler, Alfred (Jeremy Irons) and Lois Lane (Amy Adams) get any significant screen time outside of the heroes.
The biggest problem JL has is overcoming the seemingly irreparable mixed reactions to BvS, and to a lesser extent, similar reactions to Suicide Squad.
It's for this reason I've stopped reading reviews before I see a film. Particularly with comic book movies. I find the better approach is to see the film, then you have all the knowledge you need to decide which reviews actually describe the film you saw with any degree of accuracy.
For example, much has been made of Henry Cavill's CGI erased mustache that he couldn't shave due to shooting another movie during JL reshoots. I knew about this, and never noticed anything distracting about Henry's upper lip - not once. Some reviewers write about it like it spoiled the whole film.
The CGI in this film is every bit as good as any CGI you've seen in a Marvel movie - I should know, I've seen all of them and own most of them, as well as most of the DC movies too.
That's not to say the CGI is seamless, but it never is in any CGI heavy film. Stop complaining about it. It's like going to see a theatre show and complaining you could tell the sets were made out of wood and the backdrop was a painting. CGI is constantly improving but so is the audience's knowledge of being able to spot CGI filled scenes.
As I said, I enjoyed Justice League. It's lighter tone, brighter color palette, humor, and a Superman I finally recognize made it easily worth the price of admission.
Also, for the first time in a DC film, there was an end of credit sequence well worth staying for (personally I've stopped staying for Marvel's end of credit sequences after the Cap joke at the end of Homecoming... if I'm going to sit through that many credits it has to be worth while).
Justice League is a perfectly fine superhero film. If you didn't like BvS you can pretty much jump in here and pretend much of that movie didn't happen the way you remember it.
Moving forward I really hope Ben returns for a solo Batman movie (or three). Definitely on board for the Wonder Woman sequel, Aquaman movie, and Flash movies.
Most of all, I'd really like Henry to get that Superman solo movie that got hijacked by BvS. His Superman is in a really good place at the end of Justice League. Plus I really want to see the entire Man of Steel supporting cast get another crack at a proper Superman story (complete with Jimmy Olsen Junior, son of a certain CIA agent following in his dad's photography footsteps).