These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Counterpart Comparison: While none of Superman's usual Rogues Gallery shows up in any of the shorts, the Mad Scientist from the first episode greatly resembles the original Ultra Humanite only with no paralysis. Likewise, the scientist in "The Magnetic Telescope" bears more than a passing resemblance to Lex Luthor, though looks more like Doctor Sivana.
Ear Worm: Superman's theme song/leitmotif, especially if you try to sing Superman's name along to it.
Fair for Its Day: "The Electric Earthquake" has a Native American villain who is a well spokenWell-Intentioned ExtremistMad Scientist type who dressed in either a contemporary urban suit and tie or in laboratory gear, with only somewhat longer hair to mark his ethnic identity. For a 1940s American cartoon, that is a remarkably sophisticated subversion of a common racial stereotype of the time.
Fridge Logic: In "Jungle Drums", Lois is initially wearing her usual brown suit when she is captured by the natives and when she is threatened by the Nazis. Later on when she's on the pyre she's in a different white dress. Did the entirely male-populated Nazi-base just happen to have a spare white dress lying around that they decided to put her in for the ceremony?
Maybe they just removed the brown suit and left the "underdress".
Heartwarming Moments: The final short ends with Superman flying past the American flag and giving a salute.
Memetic Mutation: The opening narration, especially "Faster than a speeding bullet", and also "Up in the sky! It's a bird! It's a plane!" The fact that the cartoons were repeated all over the world as pre-film features for years after they were first made ensured that they became catchphrases for multiple generations.
In "The Underground World" Lois and the resident scientist are captured by Bird People and brought to their throne room/altar. They see a near perfect golden replica of the scientist's father who went missing in the caves where bird people live years ago. Then they get tied to a stone, the floor under them splits open revealing a pool of boiling gold liquid and they realize it's not a replica...
Older Than They Think: DC once stated in print that the earliest occurrence of the classic "Phone Booth Costume Change" happened in 1948. In the comics, perhaps — but the Fleishers were there first by seven years in "The Mechanical Monsters" (1941).
Values Dissonance: A surprising lack of this for a WW2-era propaganda cartoon, the native american villain from Electric Earthquake is even played without stereotype, but the African tribe from Jungle Drums... has about the level of racial stereotype you can imagine from this kind of media.
Visual Effects of Awesome: The rotoscoping looks amazing, and the animation still holds up well to this day. The special effects work is also particularly notable, such as the battle and subsequent meltdown of the giant laser cannon from "The Mad Scientist".
What Do You Mean, It's for Kids?: "Jungle Drums," which is loaded with Nightmare Fuel, introducing images such as barbaric natives attempting to burn Lois alive. (Amusingly, this cartoon made its way onto a "Parent Approved" home video VHS tape of Public Domain cartoons.) The poisoned needle and giant Egyptian tomb guardians in "The Mummy Strikes" are also unpleasant bedtime contemplation for nervous children.