As with every comic book movie that is part of a larger franchise, Justice League is bursting with Easter eggs and references. The Justice League Easter eggs found in the movie are some of the best the DCEU has yet offered. With so many legendary DC heroes on screen, there are tons of opportunities for references and nods to the source material. Zack Snyder didn't disappoint, setting fans to whooping and hollering at numerous points throughout the film.
While Justice League is receiving tepid reviews, comic book fans, and superhero fans in general, are apt to love it despite its flaws as a film. True DC fans, be sure to see the film before diving in - unless you don't mind spoilers and want to be able to keep an eye out. Heck, maybe even give Batman v Superman a re-watch. At any rate, be warned, there are Apokolips-tic spoilers ahead.
When the Justice League are on their way to thwart Steppenwolf, Aquaman has a seemingly out of character moment in which he starts baring his soul, talking about his fears of death and his hots for Wonder Woman. After a couple minutes of embarrassing secret spilling, he realizes he's sitting on Diana's Lasso of Truth, which compells him to open up. Hal Jordan the Green Lantern did this exact same thing in the comics.
Justice League contains a flashback to an epic ancient battle between Steppenwolf and his cronies facing off against the allied forces of Atlanteans, Amazons, Men, the Old Gods (Greek Gods), and even a few Green Lanterns (the appearance of which will make the hardcore DC fans in theaters erupt.) Since this is indeed an ancient war, the identities of these particular Lanterns are unknown (though one look's like Salaak of Space Sector 1418), and one is even slain, after which we see his power ring take off in search of a new hero to take up the mantle.
The Green Lantern Corps will return to the big screen with their own movie in 2020, which will hopefully be enough time to heal the wounds inflicted by the Ryan Reynolds fiasco.
Early in the movie, there's a scene between Bruce and Alfred in which the old butler laments this world full of new and cosmic terrors, saying something to the effect of, "One misses the days when one’s biggest concerns were exploding, wind-up penguins." This is an obvious nod to the infamous Batman villain, Penguin. It's a reminder that the DCEU's current Batman has been at this a long time and has faced villains even the most casual fans know.
When Barry "The Flash" Allen (Ezra Miller) goes to visit his father (Billy Crudup) in prison he's checked in by a guard that only the most devoted of DC movie fans will recognize. The guard is played by Marc McClure, an actor some will remember starred as Clark Kent's friend Jimmy Olsen at the Daily Planet in the 1978 film Superman starring Christopher Reeves.