Lots of Easter eggs. Many that I probably missed. The office building Superman goes into to get away from the angry crowd is the company Rex Whatsisname worked for when he got turned into Metamorpho. You probably recognize the Justice Gang's Hall of Justice from the old Saturday morning Superfriends cartoon. Interestingly, the Hall of Justice was based on Cincinnati's Union Station train terminal. And sure enough, not only did they use it for the exterior shots, they used the lobby for the interior scenes instead of just shooting them on a set.
I loved how Hoult took a page from Montalban's ST:TWOK Khan and played Lex as the hero. And from a certain perspective he has a point. And like Khan, he doesn't know when to quit and takes it too far, becoming the villain.
Was it just me or did I pick up a little young Samuel L. Jackson in Mister Terrific's mannerisms? Speaking of Mister Terrific, he's really integral to the plot and the odds that they knew the timing would line up the way it did but part of me can't help but wonder if it was a bit of a jab at the Fantastic Four's Mister Fantastic.
Last observation (for now) is the choice of supporting heroes. They went with really obscure characters. Green Lantern is kind of A-list, but instead of Hal Jordan, they went with...they didn't even use...John Stewart(?), they used Guy Gardner. Nathan Fillion voiced Hal Jordan GL and he did great as Guy Gardner. They made the character heroic, but still a bit of a dick and Fillion nailed it. Then they used...Hawkman is a pretty obscure character, but Hawkgirl probably even moreso. Then they throw in Metamorpho. But it isn't just like "here's Metamorpho," there's actually a reason for him to be there. Then there's Mister Terrific, who was great, but I'll admit I'm a comic guy and I'd never heard of him before the movie trailer came out. So, where Snyder adds Batman to the title of Superman 2 and adds in Wonderwoman, Cyborg, Flash, and Aquaman just out of nowhere, Gunn starts his universe out small, giving us something to look forward to and building it gradually. It's a different angle on the way they did it in the MCU--there each of the major characters got a movie or two before being put together in "Avengers" but the result is the same--you don't have so much going on that it detracts from the story. (IMO)