After the very real, hopeless tone of Batman v Superman: Dawn of Justice, it makes sense that the formation of the Justice League would ultimately bring a newfound sense of hope to this otherwise bleak world, and the DC Extended Universe as a whole. The Justice League has always been a positive archetype for the DC Universe and comics in general.
The first teaser poster reinforces this, with the logo and background being dull and black but the star on the logo shining brightly.
Jor El: You will give the people of Earth an ideal to strive towards. They will race behind you, they will stumble, they will fall. But in time, they will join you in the sun, Kal. In time, you will help them accomplish wonders.
Clark gives the people an ideal to strive towards in Man of Steel by stopping Zod and fighting for humankind. The stumbling and falling occurs in Batman v Superman when Bruce gives in to fear, caring less about collateral damage and going so far as to plan to murder Superman. The Justice League uniting is them joining Clark in the sun, being inspired by his Heroic Sacrifice to save the Earth from Doomsday, and their defeat of Steppenwolf is an accomplishment of wonder.
Barry Allen is the fastest man alive. He is also the fastest to accept Batman's invitation to join the league.
Diana is seen restoring sculptures (Wonder Woman showed her working at the Louvre museum before). Not only does she seek to protect mankind again, she is also concerned by its cultural heritage, such as fine art.
This is also a Mythology Gag, referencing one of Wonder Woman's origins as a statue brought to life.
How does Wonder Woman solve the bomb situation on her own? By literally smashing the glass ceiling.
The first official trailer opened showing Bruce Wayne/Batman, the second trailer opened with Wonder Woman and the final trailer opened with Clark/Superman. The Trinity that inspired the creation of the Justice League.
Batman's response to Cyborg that he's "real when it's useful" isn't just him being a Deadpan Snarker. A common complaint about most DC stories revolving around non-Batman characters is that Batman is usually so crazily-prepared for anything, that what most heroes usually spend whole stories struggling through, Batman could fix easily with all the secret weapons and plans he has, meaning that if Batman were there, the problem would logically be solved easily. Therefore, if Batman is always useful, then he's always real.
The victory shot after Steppenwolf was defeated though not killed highlight some aspect of our heroes that put them as foils of each other that become united in saving the world.
In the left, there's Cyborg and Flash doing a fist-bump. Both become who they are because of accidents, with Cyborg finally able to happily accept who he is (saving the world and being very level-headed during most of the film run prove that he's not going to be another threat to earth), while Flash finally has friends and a 'jogging' buddy that can accept who he is. Victor later helped his father on his research, while Barry actively becomes a superhero protecting Central City.
On the right, there's members of ancient civilization that included in the alliance that defeated Steppenwolf the first time. Wonder Woman leave her beloved island and family to help humanity, but got disillusioned, isolated herself (while still preserving human culture), joined the Justice League voluntarily, and regained her confidence as leader while on the team (Batman baiting the parademon reminded her of their previous talk about how a member of the team will pull Heroic Sacrifice and she's not going to let him). Aquaman was born on land because his mother wanted to protect him, he doesn't belong on either on land or sea, isolated himself (but still helping both human and Atlantean), joined the Justice League because of his duty as Atlantean king, and found a place to belong in the team (his lasso-induced Tsundere moment show that most of the rude things he said to Cyborg and others is because he's scared of dying, and of losing the closest thing he has to friends).
And in the middle, The World's Finest: Superman and Batman. The Cape and The Cowl. The hope of victims and the terror of criminals. An alien that became the best example of humanity, and a human that was forged by but ultimately rejected the worst of humanity. One is a new savior that finally changed and was accepted by the people, one is a veteran crime-fighter for 20+ years but feels like he didn't actually make a difference and is feared by people. One inspired others to be the best they could be, another gets inspired to be the best he could be (and by Nightwing existence, had been). And many others. Here are the 2 paragons that lead the Justice League to be Earth's greatest protectors.
The League revives Superman by submerging his corpse in Kryptonian birthing fluid and zapping him with the Motherbox. Superman comes back in a psychotic state before calming down. Did the Justice League create a Lazarus Pit purely by accident?
Remember the Bad Future Flash's message to Bruce in Batman v Superman: Dawn of Justice? "It's Lois! It's Lois Lane! She's the key!" It turns out she's the key to calm down an angry/confused resurrected Superman and rally him to the Justice League.
For that matter, the earliest teaser posters' claim to "Unite The Seven" doesn't quite pan out here, until you realize that Batman has subtly acknowledged Lois Lane's place among them for her role in bringing back Superman.
The Justice League is usually associated with the Greek gods. Superman is Zeus, Aquaman is Poseidon, Batman is Hades, etc. When Steppenwolf first invaded earth, the Greek gods helped fight against him and the one responsible for defeating Steppenwolf is Zeus. The second time around it's Superman that's responsible for Steppenwolf's defeat.
Diana describes The Alliance that originally defeated Steppenwolf as heroes, and in more ways than one, they were the Justice League from The Time of Myths:
Superman not only has been associated with Zeus, but as an alien, he stands for the Green Lanterns as outworlders that came in defense of Earth.
Batman represents the human tribes as the resident Badass Normal.
Wonder Woman not only stands for the Amazons, but the Olympians too due to her father Zeus taking part in the battle, as well as her divine half-sister Artemis, who is ironically her namesake (Diana is the Roman version of her name)
Aquaman represents the Atlanteans as a whole.
Ironically, the Flash is also associated with Zeus, due to his powers being derived from lightning.
Though not a member of the League, the Green Lanterns from ancient times foreshadow the presence of one of their members joining the League eventually.
After Steppenwolf's defeat, Wonder Woman notes that she's "working with children". While it does paint her as more mature thanks to Women Are Wiser, take note that Amazons are Older Than They Look - Diana's appearance as a 30-year old woman dates back to World War I and even past that, and her mother Hippolyta looks like she's only 10 years older. She is literally the oldest member of the League, having been alive way before any of them were even born; hence the comment that they're children because to her, they might as well be.
And her sly, amused smile as she says this. . . they may be children, but now they're herchildren.
Theres also the fact that her new teammates have a lot in common with her old teammates, who (other than Steve) she knew into their old age, so these new friends remind her of the earliest versions of her old friends.
Chief was a member of an oppressed people who used his exceptional skills to exceed in life. Victor Stone is a black man with a genius IQ who got a football scholarship to college.
Charlie was a hard-drinking Boisterous Bruiser like Arthur Curry, but underneath the bravado, he was struggling with a traumatic past and crippling anxiety like Barry Allen.
Sammy was an Omniglot who effortlessly assumed undercover personas. Bruce Wayne speaks many languages and blends in at an underground fight club run by the Russian mob as easily as a Metropolis gala.
Steve Trevor was an all-American Nice Guy, who despite knowing how flawed humans are never lost faith that saving them is the right thing to do, leading to his Heroic Sacrifice, just like Clark. Also, Steve was an Ace Pilot and Superman has the power of flight.
Etta was a proper English assistant who ran missions from headquarters but was perfectly capable of being in the fight, and did as she was told while being a Servile Snarker, just like Alfred.
One of the major complaints against the film was that Steppenwolf is underwhelming as a villain and can barely outmatch Wonder Woman and Aquaman, much less the whole League. But while Steppenwolf may look like an ultimate badass, his actions prior to the final battle actually indicate he isn't. In his past invasion, he was badly injured by the Olympian Gods, and actually had to be dragged away screaming by his underlings. The only reason that would happen is that they knew he wouldn't be able to handle Earth's protectors; he was dragged away because otherwise he would have gotten himself killed because he overestimates his own strength. Later, he boasts to Wonder Woman that "Those Gods [the Old Gods of Earth] died!" Except Steppenwolf had nothing to do with that, he's just trying to intimidate the only person he's met so far that stands a chance of beating him.
And for extra Fridge Brilliance: the only way for Steppenwolf to know the Old Gods died is if he's been watching Earth. In that case, he knows that the very Gods who drove him back last time were killed by Ares. Guess who killed Ares? That's right: the reason Steppenwolf keeps throwing out Badass Boasts is because he's almost afraid. And he can't afford to be afraid.
Kal hugging Lois, and pausing for a moment, to confirm that she called his mom to let her know he was back. He wasn't checking at all. He heard the rickety old truck a mile away, and was thanking her. Cue mom pulling up after about a minute.
Superman's actions in the final battle are mostly helping out the other heroes. He helps Aquaman and Wonder Woman fight Steppenwolf, helps Flash with the evacuation, helps Cyborg separate the motherbox. Even the final fight with Steppenwolf, he freezebreaths the axe to help Diana. He's clearly the most powerful member of the JL but he wouldn't have been able to do it alone. He's not a replacement for the League at all and this justifies why the League still exists after the end of the film even while Batman formed it as a replacement for Superman.
Especially the "saving the civilians" part. If Superman had been alone against Steppenwolf, he would have had to keep fighting him and the Parademons, and the civilians would have died (and the terraforming process would have been completed, all Steppenwolf and the Parademons had to do was keep Superman away from the Mother Boxes long enough). His smash of Steppenwolf bought the League breathing room to continue the battle on more even terms, then the League kept him occupied while Flash and Superman saved the innocents in the area. Superman may be the most powerful being on the planet, but even he can only do one thing at a time.
In the mid-credits scene from Suicide Squad (2016), Bruce Wayne receives personal information on Arthur and Barry from Amanda Waller, but not Victor. In keeping with continuity, Bruce manages to track down the first two, but the latter had to come to him.
Right after Superman's resurrection, Diana tries to calm him by addressing him by his birth name, Kal-El. It doesn't work. A few dangerous minutes later, Lois calls him Clark instead, and he almost instantly calms down. He then flies with Lois to the family farm, because "it's home." Supes considers himself a Kent first, since they raised him from infancy, and a Kryptonian second.
Steppenwolf can scream underwater. This makes sense since he's native to a planet that has no flora and is most likely so polluted that seawater would be less harmful on his lungs than his home planet's atmosphere.
Arthur doesn't initially get along well with Bruce, Barry, or Victor. His friction with Bruce is understandable, since he's still annoyed at Bruce for digging up his identity and tracking him down, but why does he clash with Victor and Barry? Because water doesn't mix with technology or lightning.
Superman is really cheerful in the final battle, making wisecracks and just in general being a huge contrast to how he's behaved in the climactic fights in Man of Steel and Batman v Superman: Dawn of Justice. It almost feels out of character, until you realize — the climax in the first movie was a personal, emotional fight against the last other survivor of his planet. In the second movie, it was a monster that outclassed him, after having an emotional slug-fest to the near-death with Batman. In Justice League, though, Steppenwulf is an Obviously EvilCard-Carrying Villain and Superman is way more powerful than him. In short, Superman has a fight he knows he's going to win — look at the smirk on his face when he dodges Steppenwulf's punch — against someone he can punch the daylights out of without feeling the least bit guilty. That fight is pure catharsis for Kal El, and he's enjoying it.
Look closely at how Superman fights Steppenwulf — he never follows through on a punch. His fist moves just enough to impact the other guy, then stops. So he's easily, hilariously outclassing Steppenwulf while pulling his punches. One wonders what would have happened if Clark hadn't been holding back.
This also neatly sets things up for any future confrontation with Darkseid, putting in a clear visual indicator that Superman usually holds back which will lend even more weight to however the "World of Cardboard" Speech ends up appearing in the films.
Clark's remark about coming back to life being itchy actually makes a degree of sense - wounds tend to itch when healing and if the Mother Box jump-started healing across his whole body...
Steppenwolf believes that Earth is an easy target because there are no Green Lanterns or Kryptonians to defend it, so it'll fall like the others. Which begs two very scary questions: exactly how many worlds have Darkseid's armies conquered and why is there no Green Lantern for the sector that Earth is in?
Of even more terrifying: how many Green Lanterns have gone up against the forces of Apokolips and failed?
Steppenwolf and Darkseid have now seen the error of using fear-hungry parademons to conquer planets. Now Darkseid will be more motivated than ever to discover the Anti-Life equation and put humans under complete control. Final Crisis anyone?
Batman's decision to use the Mother Box to bring Superman Back from the Dead is openly discussed for its ethical ramifications and the potential risk of Superman coming back wrong, but Batman insists that they go ahead even if there's a "fraction of a chance", and his backup plan in case of Resurrection Sickness is using Lois Lane, more or less as "bait", as a Living Emotional Crutch. This ends up working out for the best, but consider that Superman in his brief rampage unleashes laser vision on the League, hits a cop car, with the officer diving out, and that he was on the path of killing the League and Batman. It could have easily become tragic, and a Zombie Clark might have killed Lois too in a mindless rage. Basically, Batman was really putting the lives of the League, and Lois, and other bystanders on the line to bring Superman back, for what was essentially a huge risk and gamble. The fact that it worked out for the best is Moral Luck, and his motivations for doing so, and his brushing aside Diana's complaints about how leaders have to ask people to die for them and command Canon Fodder suggests a lot of dark stuff about Batman's character, and his past with Robin and his general Knight Templar ethos.
Leads to Fridge Brilliance. The League as it is has already lost to Steppenwolf once at this point and he's close to unleashing a weapon that will destroy all life on the planet. Batman figures they have nothing to lose, either the attempt to resurrect Superman fails and they're probably screwed, Superman is resurrected as an insane zombie berserker and destroys a city or two before Steppenwolf destroys the planet anyway, or he's resurrected as sane and can help stop Steppenwolf. Regardless, they can't really get much worse off trying at this point than they already are.
Remember in Wonder Woman how only a few German soldiers were able to deal some heavy casualties to the Amazons? This time a even greater Parademon amount descends upon Themiscyra and inflict even greater losses to them. Its very hard to believe that Phillipus came out alive of that dome along with the other Amazons.
Also, while the Amazons in the passage leading to the dome avoiding being killed by Steppenwolf and his parademons, they have no escape and were doomed to die of dehydration and starvation.
Steppenwolf's threats about making all the Amazons "love him" carry some really creepy implications about him turning them into his own personal harem. Dirty Old Man indeed...
Wonder Woman's exposition to Batman regarding Steppenwolf and who he is, particularly the origin of the Parademons basically being the fate of innocents or those who opposed him in the past. In essence every single Parademon they're dealing with may very well have been someone else (or worse, someone they knew) before they were turned.
Martha Kent's scenes individually are played for drama or laughs. She loses the farm because she missed a payment or two. She refers to Lois as thirsty (which would be insulting if it was intentional) instead of hungry. She asks "It is really you?" when she sees Clark again. But taken together, they imply that something might be wrong. Maybe early onset Alzheimer's. Or maybe something Lex did to her while she was kidnapped...
She experienced a lot of stress after her husband and son died. After she lost the house, her level of stress likely increased due to her adapting to her new life in a new place as a widow and a waitress. She may be so stressed that she is starting to forget things.
When we see Lex in the post-credit scene, he's surrounded by attractive women. Their poise suggests that they're bodyguards, but they don't appear to be armed and are just as likely there to serve equally as eye candy. Though they might have signed up willingly and are being paid well to keep Lex company, keep in mind that as Batman V Superman demonstrates, he has connections to the Russian mob, who in turn have connections to sex traffickers.
During the ending scene when Lois is narrating over the various heroes, one of the scenes is Wonder Woman in England, packing up some recovered museum pieces while the criminals are spilling their guts out to the police officer about all the heists they pulled. While that would help in recovering lost or stolen treasures, the testimony of the criminals, at least in the U.S., would be inadmissible in court. No one can be compelled, even by a superheroine's magic lasso, to give evidence against themselves. The entire confession would be thrown out of court.
It may not be for use in court, so much as to help the cops narrow down the who's who of crime groups. So they'd know that that particular group worked for, say, Sionis, and could easily start tracking patterns and methodologies