Welcome to Atlantis
A large part of Justice League's allure lies in its new characters, who while teased during Batman V Superman: Dawn of Justice, make their proper DCEU debut in this film. Among the newcomers is Arthur Curry AKA The Aquaman. He's a loner, an outcast, and he downright refuses Bruce's initial invitation to join the team because, as he says (or rather quotes of Friedrich Schiller), "a strong man is strongest alone." Obviously, that is just the kind of sentiment Justice League aims to prove wrong.
Once Arthur does decide to join, he needs to pick up a few things from Atlantis, but upon his arrival he finds Steppenwolf has come to take back the Mother Box held there. They fight, but Steppenwolf still manages to escape with the Mother Box. It's a scene which, while important to the film's plot, is mostly here to offer up our first real look at Atlantis. It's a brief visit to the underwater kingdom, revealing a landscape dotted with sunken ruins and the Mother Box held in something of a submerged tower.
What's more telling is that this scene appears to be the first meeting of Arthur and Mera - the future King and Queen of Atlantis. Mera's shown guarding the Mother Box, displaying a little of her hydrokinesis, and it's implied that Arthur hasn't been seen in Atlantis for a long time, though she still recognizes him as Queen Atlanna's oldest son. We also get a glimpse of how the DCEU will handle underwater communication, with Mera creating a pocket of air for her and Arthur to converse inside; a change from the comics in which characters just speak normally while underwater, but a necessary one for any chance at a believable, live-action adaptation. Expect the first trailer for 2018's Aquaman to reveal more, but this was at least a little taste.
It is both the biggest reveal and the least surprising moment of the film, but at about halfway through its runtime, Justice League resurrects Superman. This isn't exactly a spoiler, with press for the movie ranging from coy comments to outright confirmation of his return, but the moment itself is exciting nonetheless.
After debating the pros and cons of resurrecting the dead hero, the plan is eventually finalized - they'll submerge Clark's lifeless body in the regeneration chamber within the Kryptonian ship (the very same that Lex Luthor used to create Doomsday) and use the power of the one Mother Box they've recovered to revive him. When Superman died fighting Doomsday in the comics, he was later revealed to have only been in a death-like hibernation and was revived thanks to the solar energy stored within cells, but Justice League goes for the full death and resurrection route. And once the deed is done it appears as if Kal-El is back, looking just the same as ever - but is he really the same man when he returns?