- Anvilicious: The title Doomsday Machine kind of says it all along with Kirk's theory of the planet killer being an ancient weapon in a war.Kirk: Built primarily as a bluff. Never meant to be used for it's so powerful it would destroy both sides.Kirk: In the twentieth century, the hydrogen bomb was the ultimate doomsday machine and we used something like it to destroy another doomsday machine. Probably the only time such a weapon was used for constructive purposesSpock: Appropriate, Captain. However, I can't help but wonder if there are more such devices wandering the universe.Kirk: I should hope not. I found one quite sufficient.
- Naturally, this episode was An Aesop criticizing the Cold War arms race. And the fact that the superpowers thought that the fear of Mutually Assured Destruction was a deterrent to nuclear war. Both sides also had enough nukes to destroy human civilization many times over. This was a rare instance where anyone in universe explicitly pointed out an analog to real life.
- Ensemble Dark Horse: Although he only appeared in this episode (well, obviously) his haunting story and William Windom's brilliant acting has made Decker one of the most memorable characters in the entire original series, and its status as a fan favorite is largely because of his contributions. His character was memorable enough that William Decker, Matt Decker's son, was created for Phase II, which would be transformed into Star Trek: The Motion Picture.
- Ham and Cheese: William Windom thought the episode "seemed kind of silly, with the planet eater and the spaceships", and acted as though he was playing in a cartoon.
- Some have noticed Windom's exaggerated accent in some scenes and decided that Decker must be from "Bahston", Massachusetts.
- This backfired in that it gave his performance as Decker the intensity and unhinged lunacy it needed to become truly memorable and effective. Today it's considered one of Windom's most iconic roles; upon his death, many obituaries reflecting on his career led with Decker even over his Emmy-winning turn as James Thurber or his long-running supporting role on Murder, She Wrote.
- Harsher in Hindsight: In Star Trek: The Motion Picture, Commodore Decker's son, Capt. Will Decker, would have his own encounter with a destructive space machine ending with his demise (maybe).
- Hilarious in Hindsight: Also a bit of Older Than They Think.... Let's see. The villain is a giant, mindless eating machine that can destroy just about anything in its path. Regular weapons have little effect on it. A major character is a ship's captain whose obsession with killing the eating machine puts other people's lives in danger, and his final fate is to be killed by the villain. The villain is killed, basically, by throwing a bomb into its mouth. And, last but not least, the villain is accompanied by a simple but very distinctive musical cue. (Dah-dum dah-dum dah-dum dah-dum dah-dum....) Gee, I wonder where we would see these things again...?
- The planet-killer is a massive, nearly invincible spaceship appearing several miles long, that dwarfs the Federation ships and, well, eats planets...looks and sounds remarkably similar to the Narada, right down to being destroyed by a smaller ship loaded up with an explosive payload launched down its gullet by a pilot who beams out at the last moment.
- I Am Not Shazam: The planet killer is consistently referred to as just thathowever, it is sometimes remembered as "The Doomsday Machine", that being the name of the episode and everything. The title refers to Kirk's theory that this is a doomsday machine from some ancient war long after the builders are gone. In truth, we never really learn if that is the case...or something completely different.
- Jerkass Woobie: Decker. We feel bad for him since he's mourning the loss of his crew and clearly suffering from post-traumatic stress disorder. Still, he can be a real jerk.
YMMV / Star Trek S2 E6 "The Doomsday Machine"