"Who Watches the Watchmen?" is written on the wall of a building.
Under the Hood can be seen in a bookstore window.
A gun-wielding Fat Albert and the Cosby Kids are shown running away from a Bat-Bot, then getting surrounded by them.
Green Lantern's Emerald City incorporates the ELO spaceship as part of its design.
The Beatles can be seen playing in the club in #2—sort of. From the knees down. There was also an attempt for Sgt. Pepper Humongous Mecha in the final battle scene, but that reference is nearly too subtle, even with annotation.
In the same panel as the Beatles, Tommy and the Acid Queen can be seen at the far left of the panel.
When a huge number of super-powered people are charging in the gulag rebellion, several tiny Marvel characters can be made out, including Captain America, Spider-Man and Thor.
Storm can be seen in the bar scenes.
As can Rorschach.
The Wonder Twins are also bartenders.
The Bjork poster.
Two background characters, April Fool and Goblyn Lord, are based on Columbia and Riff Raff from Rocky Horror Picture Show.
The Bat-Bots are modeled on the mecha from Patlabor. Batman's armor itself resembled RoboCop.
One of many cameos, a Background character in the gulag battle was visually based on XO-Manowar of Valiant Comics. At around the same time as Kingdom Come was being published, Mark Waid was also signed on the write the Acclaim Era Reboot of the character, XO-Manowar: Rebirth.
While Magog is mostly based on Cable, he has elements of other Rob Liefeld characters as well. Most notably, his helmet is based on that of Shatterstar's with the addition of horns.
The center section of the Gulag is designed to look like the Hall of Doom from Challenge of the Superfriends, and the UN building looks like the Hall of Justice.
Captain Marvel Jr looks just like Elvis Presley. A Recursive Adaptation in that Elvis originally modeled his style after Captain Marvel Jr.
In his brief appearance, The Joker looks like an aged Jack Nicholson from Tim Burton's 1989 Batman movie (by coincidence, he also comically looks a bit like Silvio Berlusconi).
The minor character Tokyo Rose resmebles a Japanese-themed Chun Li.
Phil Sheldon from Marvels sits next to Norman and The Spectre in the epilogue.
He's also among the crowd of newspeople during the Justice League's press conference.