Batman insinuates that Ace has replaced Harley in Joker's affections. Ace's voice actress, Hynden Walch, went on to voice Harley's character for The Batman (2004).
Ace is based upon comic book super-villain Psycho Pirate, a telepath who has the ability to induce insanity in others by looking at them.
This is the last episode in the Justice League series where Mark Hamil voices Joker. Joker does not make an appearance in Justice League Unlimited, but Mark Hamil does provide the voice of James Jesse, aka The Trickster in one episode of JLU.
Except for the group photo that Joker pulls up, this is the first storyline in Justice League where Martian Manhunter doesn't make an appearance.
Included as a bonus cartoon on the bluray release of Justice League: Doom (2012).