The use of "Frère Jacques" by Batman to keep Dee out is somewhat a joke which pokes fun at Destiny's powers. The reason behind this is the translation of "Frère Jacques" from French to English is roughly: Are you sleeping, Are you sleeping, Brother John? Brother John? Morning bells are ringing, Morning bells are ringing. Ding, dang, dong. Ding, dang, dong.
One of the signs that actually make sense (if not to an English reader) in the city now foreign to Green Lantern says "Humuhumunukunukuapuaa", which is Hawaii's state fish.
Many fans pointed out that Dr. Destiny's costume bears a strong resemblance to Skeletor, the main villain from the popular He-Man franchise. Although Dr. Destiny's character predates He-Man by several decades, the show's writers tried to avoid the comparison by changing the color of his costume. However they decided that a green cloak made him resemble Dr. Doom, the Fantastic Four's enemy, while a gray one made him resemble the Phantasm from Batman: Mask of the Phantasm, and they felt too many of the show's villains had purple as their main color already (Joker, Luthor, etc.) So they kept the blue cloak from his comic book appearance.
Superman greets Jimmy as "My pal" before inadvertently killing him, a joke-reference to "Superman's Pal (1999)", which was unpopular even eventually among the writers.
The first two people The Flash tries to speak to resemble Dick Grayson and Barbara Gordon (Nightwing, formerly Robin and Batgirl, formerly Oracle, respectively).