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.
Ear Worm: Superman's theme song/leitmotif, especially if you try to sing Superman's name along to it.
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?
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).
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.