Angst? What Angst?: Rick fails to save his mother, and she is executed right before his eyes. Five minutes later he's perfectly fine, and laughing at Zuffy's antics. Then he learns that her memories and personality were imprinted on a crystal. He cheerfully decides to use her as a CPU. Flash is perfectly cool with this as well.
Anvilicious: In a few episodes, the kids ended up realizing how drugs and alcohol are really, really, really bad things.
Complete Monster: Prince Kro-Tan is the equally-horrible son of Ming The Merciless. Kro-Tan secretly hates his father and plans to overthrow him, leading him to get some magic powers. Kro-Tan manipulates the Defenders of the Earth into helping him activate a device that he uses to vaporize his father and then plans to let it kill the Defenders of the Earth. Kro-Tan tries to conquer Earth like his father before but eventually gives up and instead plans to launch a missile at Earth to completely destroy it. To reclaim dominance over his father, Kro-Tan enslaves several people in the mines on Mongo and forces them to work nonstop, not caring if they end up dying from the brutal labor. Kro-Tan does this so he can block out Earth's sun and cover it in an eternal winter.
Foe Yay Shipping: In "The Golden Queen, Part 1", a villainness approaches a sleeping Jedda and comments on her beauty. Censors didn't notice. Wanna read something even more squicky? Said villainness had just married the Phantom.
Heartwarming Moments: The episode Starboy had an all powerful child who had nearly wiped out a civilization. Due to this, an entire alien race tried to convince the Defenders he was too dangerous to be alive and must be killed. The Defenders answer to this? Befriend the child, playing games with him and teach him the value of helping others and making friends of his own.
Older Than They Think: This series was not the first animated teaming of the four adult heroes. In the early 1970s, the Saturday morning ABC Saturday Superstar Movie showed a Crisis Crossover between a large amount of newspaper strip protagonists in Popeye Meets the Man Who Hated Laughter. When the villain seeks to eliminate humor by kidnapping and imprisoning all the funny characters, the action characters are sent by the President to rescue them. These include Flash, Mandrake, Phantom and Lothar among others. Perhaps also foreshadowing this series, they are all saved by the funny characters showing their captor the error of his ways.
Spiritual Successor: Dynamite Comics have reunited Flash Gordon, The Phantom, Mandrake and Lothar in their Kings Watch series. However this time it's not set in the future, there's no children of the original heroes and the threats faced are mostly supernatural rather than alien. This series has lead to a Shared Universe of titles under the King banner featuring solo titles for the aforementioned three (with Lothar as the new Phantom), plus Prince Valiant and Jungle Jim.
Tear Jerker: Dale(?note It's never confirmed directly, at least not in the show itself.)'s death and Flash and Rick's reactions to it.
