Superman was originally a Flying Brick who couldn't fly, only jump (hence, "able to leap tall buildings In a Single Bound") and run (hence, "faster than a speeding bullet"). Thanks to Serial Escalationnote and Fleischer Studios, as it was easier and cheaper to animate a man in flight than a man continually jumping, though, he could already fly higher and faster than most airplanes well before America got into World War II. His invulnerability level changed too, since his first origin story said that only "an exploding shell" could hurt him, while these eventually became ineffective as well, necessitating the invention of kryptonite. He also lacked many of his current powers, sported a different (and often inconsistently illustrated) costume, killed bad guys, and was something of a rebel. But especially weird, given his current international sensibilities, is when we're told that "Superman says 'You Can Slap a Jap'" as seen on many World War II-era covers.
Superman wasn't always the lawful, pacifist hero he is known as today. In fact, he arguably caused more harm than good in his earlier comics, since he was actually kind of nuts. Examples are deliberately trapping a party of rich people in a mine to demonstrate the mine's safety problems, and drugging a football player and taking his place on the team, so that an opposing coach, who is guilty of cheating, won't win. He also once tackled teenage delinquency by demolishing their slummy neighborhood, making sure that the residents had evacuated with their possessions, upon learning that the government would be obligated to rebuild the neighborhood. Then there was that time he started to smash up cars to force the government to put in more stricter traffic laws. A good example of how much his characterisation has shifted is how both Superman Returns and Superman & Loischanges the famous scene of him lifting up a car to have him saving an out-of control driver. In the original image, he was about to use it to club a criminal to death.
Superman originally had much less of an attachment to his Clark Kent persona, with some of the early radio serials even saying it was completely made up, and he would complain that he wished he could be Superman all the time. This was later retconned with the introduction of "Ma and Pa" Kent, and his identity as Clark Kent evolved to become as important to him as that of Superman, culminating in a 1945 story had him say he needed to be both to feel whole.
Clark Kent's "mild mannered" personality used to be completely be an act, with Clark deliberately acting like a coward in all aspects of his life to throw people off the idea he could be the heroic Superman. Most modern stories instead portray Clark as an Intrepid Reporter, with him being genuinely awkward and unsure of himself when he's not not in costume.
Lois Lane originally had a very negative impression of Clark Kent, who she regularly criticized for being a coward and unmanly. She would constantly compare him unfavorably with Superman, much to Superman/Clark Kent's chagrin. With the modern versions of Clark usually being more heroic, or at least significantly less of a deliberate coward, Lois is often implied to have a crush on him long before she figures out he's Superman.
Clark Kent and Lois Lane didn't originally work for the Daily Planet in the fictional Metropolis. Instead, they worked for a newspaper company whose exact name was often ambiguous, at one point even being the Evening News in Cleveland (Jerry and Joe's hometown). Usually, though, when it was named it was called the Daily Star, before switching to the Planet more or less for good in 1940.
Originally Superman's only interest in being a reporter was using it to get advanced notice of potential crimes. Today he's much more likely to have a real interest in journalism, and see it as another way to help the people of Metropolis.
Initially, it was said that Superman's fantastic abilities were inherent to all indigenous Kryptonians.note One of the first stories, if not the first, to explore the exact events that led to Superman/Kal-El being sent to Earth, had Kryptonians dismissing Earthlings as being 'hundreds of generations behind them' and that 'they didn't even possess X-ray vision!' The same story has a Kryptonian couple worrying about their son's lack of progress learning calculus, with the husband observing that soon he'll be three years old and shouldn't have problems then. Subsequent re-tellings would establish that this was not the case, with Superman's powers instead being explained as the product of Earth's yellow sun and weaker gravity - although they still apply to other Kryptonians.
For the first 10 years Superman was published, neither he or anyone else knew that he was an alien from the doomed planet Krypton. His origin was never questioned in-universe. Since then, it's become a very important part of his self-image and the way he's seen by others on Earth.
In 1940's story The K-Metal from Krypton, Kryptonite is not called Kryptonite yet but K-Metal, and its effects are significantly different from its future appearances. Just for starters, it nullifies Superman's powers rather than killing him, and humans gain Kryptonian powers upon touching it.
In one of the earliest strips that details Superman’s origins, it’s shown that Jor-El (and by implication, the rest of the Kryptonians) could demonstrate feats of strength and speed comparable to his son. This was back when Superman’s powers were natural components of a more “evolved” race, before it was established his powers came from Earth’s environment.
In his earliest appearances, Lex Luthor was a war profiteer whose scientific genius was considered "mad" specifically because he used it to invent horrific weaponry to sell to the highest bidder. Later, he became a Mad Scientist who uses his skills to commit crimes. It wasn't until the Iron Age that Lex Luthor became a Corrupt Corporate Executive who moonlights as a supervillain. During the Silver Age, his motivation for battling Superman was actually more complex: In How Luthor Met Superboy, Kal and Lex became friends. In gratitude for setting up a lab, Lex discovered a cure for Kryptonite, but he accidentally started a fire. In order to save him, Superboy used his super breath to blow out the fire, accidentally killing Lex's experiment (and causing Lex's baldness). Lex accused him of intentionally destroying his experiment -an artificial lifeform- out of jealousy, decided to get even with the "backstabbing" alien, and the rest is history. Before that, his Silver Age relationship with Superman boiled down to a simple villain-who-gets-foiled-by-the-hero dynamic.
Even Lex's modern incarnation presented Early Installment Weirdnesses. Originally, Lex had little interest in keeping his clean view in the public eye; in fact, he wanted everyone to know how rotten he was, as him eluding arrest had little to do with him being a Villain with Good Publicity, and more with the fact he basically owned Metropolis, and could force authorities to turn a blind eye to his illegal activities.
The magazine wasn't born as a Superman/Batman crossover series, but as an Anthology Comic, wherein Superman, Batman and other heroes like Zatara, Sandman, the Star-Spangled Kid, Green Arrow, Lando or Red, White and Blue lived solo adventures. Initially, Batman did not even have top-billing, his strip usually having the last slot. Superman and Batman's first joint adventure appeared thirteen years later in issue #71 (July, 1954).
Judging by the covers, the World's Finest were Superman, Batman and Robin, who was featured in all covers until issue #85 (December, 1956).
In his first appearance, Brainiac is remarkably small, scrawny and big-headed compared to later portrayals (possibly an instance of differing artistic styles, since Curt Swan's cover art is consistent with said posterior depictions). It's also implied that Brainiac is an organic being, whereas later stories would reveal he is a humanoid robot.
Brainiac claims to be stealing alien cities to repopulate his home world after a plague wiped out his then-unnamed people. Later stories would retcon Brainiac into being a living computer spy created by the computer tyrants which ruled over planet Colu before being overthrown and destroyed during a rebellion.
Superman takes the Bottle City to his Fortress, placing it in a niche excavated in a wall outside the fortress. Later comics would have Kandor kept deep inside the Fortress and linked to complex life support systems.
Back then, Brainiac was just an alien scientist who collected cities in bottles. It wasn't for six years that it was established that he was a super-computer, a characterization that has stuck ever since. It wasn't some time after that that we saw full use of the fact that he was an AI. At first, he and Lex (in his own original mad scientist persona) were basically the same character, and they teamed up a lot. When the makers of an educational computer/toy by the same name complained, he was retconned into being a computer, with an ad for the aforementioned computers thrown in.
Brainiac has a white alien monkey that serves as a sort of Right-Hand Cat for him; Koko has rarely ever appeared since, and often in an unrecognizable form (in Superman: Brainiac he is a baboon-like monster).
In his first story, "The Super Dog from Krypton", Krypto looks mostly like a cartoon dog thanks to his Pluto-like huge round eyes. When he became a permanent cast member, Krypto was quietly redesigned to look like a real Earth dog, usually a white Labrador retriever (although he has also been depicted as a spotless Dalmatian, or even a dire wolf).
"The Phantom Superboy" is the debut of the Phantom Zone, but ironically nothing is really seen from the Zone. Although Superboy is sent into the Zone, he floats around the solid world as an invisible, intangible ghost who cannot affect nothing except by projecting his thoughts. And he never runs into any inmate, although he guesses they must be still alive. Later stories would flesh out the concept, depicting an icy-cold, indigo-hued voidness populated by dangerous vindictive ghosts who are constantly watching the physical world.
Prince Adam shows up for the first time, and his secret identity is depicted as a boisterous, loud braggart who cannot take anything seriously, rather than an unrealiable, foppish princeling. His He-Man identity is armed with one axe instead of the Power Sword, and he doesn't transform into He-Man by screaming his iconic catchphrase "By the Power of Grayskull!" Instead, the Sorceress gives him and Cringer a magic zap that transforms them when they enter her hidden sanctum.
Man-At-Arms also makes his first appearance. He's young and lean instead of a bulky middle-aged man, he ignores He-Man's secret identity, and he lacks his iconic, bushy moustache.
Sorceress is called "Goddess", is not wearing her iconic bird costume, and dwells in the Cavern of Power instead of Castle Grayskull.
Skeletor wields one half of the Power Sword but does not wield his Havoc Staff.
In the second MotU crossover "Fate Is The Killer", the King and Queen or Eternia are white-haired elderly folks. A few months later, the cartoon would introduce them as brown-haired middle-aged people, which has been their default depiction since.
Supergirl being younger than Superman is bound to surprise comic fans accustomed to the modern versions who were born fifteen years before Kal-El, whom they regard as their "baby cousin".
Kara's name is... Kara, plainly. She would not be given a surname until Action Comics #288, which came out three years later. Similarly, her mother was not named in this story. Later she was named "Allura", which eventually morphed into "Alura In-Ze".
Kara's birthplace was "a street of homes" which survived Krypton's explosion -and retained a bubble of air- by sheer and unexplainable luck. After several retellings it became Argo City, which survived Krypton's destruction because it was protected by a force dome.
Neither Zor-El nor Alura know anything about Earth or Superman, other than the former being a livable world and the latter being another Kryptonian survivor. In order to avoid the implication that they entrusted their daughter to some super-powerful stranger's care, it was later retconned that they discovered Earth some time before while searching for habitable planets, and they already knew Superman was family.
Superman throws his only living family member in an orphanage right after meeting her, in contrast with future incarnations who at the very least want to take her in but are prevented from doing so by external factors.
When Kara Zor-El was brought back in The Supergirl from Krypton (2004), the first years of her Supergirl (2005) book were plagued with weirdness due to editorial carelessness and a poor understanding of the character: Kara was turned into a mood-swinger angst-filled and jerkass Anti-Hero, her personality changed every story, her backstory was constantly retconned and she displayed a ridiculous power of sprouting crystals from her body. After issue #20, she was given a heroic, likable personality and a definite backstory, and the early characterization and weird powers were retconned out and ignored.
"The Super-Steed of Steel": In his introductory storyline, Comet developed a crush on Supergirl and gained the power turn into human each time a comet passes through Earth's Solar System. His following appearances would completely ignore both his crush on Kara and his alternate human form.
In their first story, the Legionnaires wear their names written on their uniforms, and Cosmic Boy has a "Super Hero Club" yellow emblem sewn on his shoulder. They fly around by using jet packs, no Flight Belts or Rings. And they will wear completely different suits in their next appearance (Adventure Comics #267).
Cosmic Boy's eyes shoot magnetic beams, and his powers were given by a super serum. Later it would be retconned that magnetism control is his race's innate power.
Garth Ranzz's codename is Lightning Boy. Right afterwards "Boy" would be changed to "Lad". He also needs to clap his hands together to hurl lightning bolts.
Saturn Girl is a redhead. From her second appearance on, she would be blonde.
When Saturn Girl, Lightning Lad, and Cosmic Boy first met Supergirl (Action Comics #267), they told Supergirl they were the CHILDREN of the Legionnaires who met Superboy in their first two appearances. In reprints of the story, this dialogue was changed so that they were now the same Legionnaires.
The Legion HQ is located in Smallville. The next stories will quietly move it to Metropolis.
The Legion hails from the 25th century. Later, it would be retconned the team was formed in the 30th century.
