Author Appeal: According to Frank Miller's intro to the graphic novel, the entire story grew from his refusal to accept that he could be older than Batman, who he saw as a perpetual father figure. Up until this point, Batman was always 29. The first comic book was released when Miller was also 29.
Follow the Leader: This miniseries singlehandedly ushered in the new age of Cranky Batman.
What Could Have Been: A bunch of things were changed from the original draft of the climactic battle:
Green Arrow was absent, and instead it is Bruce who releases the Kryptonite Gas from his suit. There was also a moment where the sky momentarily opens up and reveals the sun, briefly healing Superman and thereby avoiding the Curb-Stomp Battle that we all know and love. Bruce also taunts him about Lois to engage his Berserk Button, which ultimately leads to his defeat. In this version, however, Bruce barely survives the battle and ends up in a intensive care, where Federal Agents anxiously wait to question him. Alas, Bruce manages to swallow a pill hidden inside his arm that allows him to fake his death and to continue to work undisturbed. The final shot would've emphasized the theme that while Bruce Wayne dies, the Dark Knight lives.
In the book itself, it's only implied that the Joker killed Jason Todd. Rumor has it that not only was that fact going to be originally outright stated in the text but that it'd be stated the Joker raped Jason before he killed him. Supposedly, DC stepped in and made Frank take it out. However, even so, Frank Miller's Joker has been stated anyway to be a rapist in further works set in the Dark Knight Universe, aside the hint that he may have raped Selina Kyle after mind-controlling her.
While the book ends with Batman faking a heart attack, symbolizing that Bruce Wayne has died but the Dark Knight lives on, Frank Miller revealed at the London Comic Con 2018 that his original ending was more a The Hero Dies-one in which Batman would have been gunned down by the GCPD members led by Commissioner Ellen Yindel.
From the DC Animated Universe: In addition to Alfred, Jean-Claude Sachot as Commissioner Gordon and Emmanuel Jacomy as Superman/Clark Kent.
From Batman: The Brave and the Bold: In addition to Alfred and Superman, Xavier Fagnon as the Jokernote except for Part 1, where his only line of dialogue is done by Jérôme Pauwels and Kelvine Dumour as Selina Kyle.
From Young Justice: In addition to Gordon, Alfred, Superman, and the Joker, Jérôme Pauwels as Oliver Queen.