During the early years, DC's different superhero stories very clearly took place in separate continuities. An issue of Batman from the 1940s, for instance, had Dick Grayson get an autograph from Jerry Siegel, identified by name and explicitly noted as "the creator of Superman".
In Wondy's first appearance in All-Star Comics #8, the introductory blurb about her powers is "As lovely as Aphrodite, as wise as Athena, with the speed of Mercury and the strength of Hercules"; all later iterations of this blurb will end with "faster than Mercury and stronger than Hercules".
She didn't have her iconic Lasso of Truth for her first few appearances, and was only given it by her mother in the sixth issue of Sensation Comics. Also, it was simply known as the "Magic Lasso" in the early days, and compelled captives to obey Wonder Woman's every command by imposing her will on them. The idea that its primary function was to force people to tell the truth originated in Wonder Woman (1975), and didn't make its way into the comics until George Pérez rebooted Wonder Woman's origin after Crisis on Infinite Earths for Wonder Woman (1987).
Likewise, Wonder Woman couldn't originally fly, which explains why she needed the Invisible Jet in the first place. And it wasn't the Invisible Jet at first, either, but simply the Robot Plane (with standard-for-the-time propeller propulsion, which somehow worked in space).
The Golden Age Wonder Woman stories come off as rather kinky nowadays, with a lot of gratuitous Bound and Gagged scenes, even by the standards of the era. Also, Wonder Woman's major weakness was that she would lose her powers if her bracelets were ever welded together by a man, a problem she obviously no longer has.
The Amazons were all white in the Golden Age stories and for much of the comic's history. It was George Perez who decided to give Paradise Island a racially diverse population, as he felt there were some invokedUnfortunate Implications behind having an all-female utopia consist solely of white women.
There was a greater focus on psychosexual and gender themes in the Golden Age stories. In addition to the ubiquity of the aforementioned bondage themes, Wonder Woman had several villains who were female crossdressers, like Doctor Poison and Blue Snowman.
Wonder Woman had a lot of rather silly Golden and Silver Age villains who are pretty much never used any more, such as Mouse Man.
It helps that the original creator was heavily into bondage, and worked it into his stories whenever he got the chance. Later writers didn't share the fetish and so it was toned down heavily even before the Comics Code.
Shazam!: Early comics treated Captain Marvel as Billy Batson's distinct Super-Powered Alter Ego rather than Billy in an adult body with superpowers. When Roy Thomas rebooted the character's origin after Crisis on Infinite Earths, he introduced the idea that Billy and Captain Marvel were one and the same, with the Captain now having a more naive and childish personality at times. This depiction was cemented in Shazam!: The New Beginning and Justice League International, and has basically become the character's default portrayal across all media since then (such as in Justice League, Batman: The Brave and the Bold, Young Justice and the live-action movie). Only a select few works, such as Jeff Smith's Shazam! The Monster Society of Evil, have retained the original Golden Age concept of Captain Marvel being a separate entity.
Lots of members of the Justice Society of America were completely different. In fact, Pre-Crisis, many "modern" superheroes like Batman and Superman coexisted with them and were members of the team rather than its successors. These differences and changes were either retconned away or explained as having been a previous member who retired or died during the timespan between the JSA's initial disbanding and the formation of the Justice League. To whit:
The original book as a whole serves as one for team books. There were short stories in each issue that had the various members of the JSA hanging out together, but this was little more than a framing device; every other story would be about the members of the team operating solo. It was functionally an anthology book rather than a team book.
The first Atom didn't have the character's signature size-changing powers, he was just a guy with Super-Strength and an "atomic punch" who was a bit shorter than average. His costume was also totally different, with a yellow shirt and a cape.
While not a Golden Age character, the Atom's godson Atom Smasher used to call himself Nuklon, and his hair was cherry red rather than darkish brown. Also he had a Mohawk. Yes, it was exactly as ridiculous as it sounds (it actually became something of a Running Gag for the series, with nobody letting Al live it down).
Originally Doctor Fate's magic wasn't based on Order, as it's most frequently been post-Crisis; it was merely general spiritual power. This extended to his archenemy Mordru, whose powers weren't based in chaos and who wasn't immortal. During his first meeting with the JSA, Fate also claimed that he had no childhood and was actually an Artificial Human created by the Elder Gods. His origin story, told the following year, would completely contradict this plot point by showing that Kent Nelson was indeed a naturally-born human prior to encountering Nabu. _T
In his first few appearances, The Jay Garrick Flash had no secret i.d. He was called by friends and co-workers as Jay even when in costume. And his speed, though superhuman, was more around 100 miles an hour, not the thousands of mph needed to circle the earth or run on water.
Hourman's Super Serum used to be completely beneficial and harmless, in stark contrast to how it would later be portrayed (addictive and dangerous, with Hourman taking years to overcome it and create a non-harmful version).
The Spectre was actually a member at one time and not only that, but he was just a fairly typical superhero. No horrific murdering of criminals, no inability to understand mortals, and he and Jim Corrigan were the same person, not two separate entities sharing a body. Also, like the Batman story mentioned above, an early Spectre story pretty strongly implied that he did not exist in a universe where other heroes like Superman were real. The issue in question featured a journalist who bore a strong resemblance to a certain mild-mannered reporter, with a detective even telling a patrolman to "keep an eye on Clark Kent here, keep him from getting into trouble." An additional Lampshade Hanging occurred when the patrolman asked the reporter "Are you really Superman?"
The first Black Canary didn't gain her famous sonic scream power until 1969, with prior JLA/JSA team-ups instead making her a little more “super” by having her use special weaponized pellets (such as one that froze opponents and another that caused paralysis) hidden in the amulet worn around her neck. She also started out as a villain, in that she was a thief who stole from other thieves. The first Black Canary was originally a supporting character of Johnny Thunder but she ended up becoming a Breakout Villain who surpassed the actual protagonist of her comic. By the time she became the lead of her own comics, her original love interest Johnny Thunder had been replaced with her definite love interest (and the father of the post-Crisis Black Canary) Larry Lance.
The first Red Tornado is probably the most striking example. For one thing, it was a woman posing as a man and she was completely human rather than an android. She also didn't really have that much red on her costume and had no superpowers or gadgets to speak of, relying on her own wits and strength.
While not a Golden Age character, Stargirl was much meaner and more disrespectful in her initial appearances. She also did not have the name "Stargirl" (instead going by the moniker "Star-Spangled Kid") until over four years after her creation, and didn't have her trademark cosmic staff until it was given to her by Jack Knight at the end of the Starman series. In her original series, Courtney was more of a melee fighter, with her cosmic converter belt giving her physical abilities like enhanced strength and speed, as well as the power to blast shooting stars.
In Aquaman's original Golden Age adventures, he was a normal human whose powers came from ancient Atlantean techniques taught to him by his father, a famous ocean explorer. Aquaman's base of operations was a sunken boat, Atlantis was a dead kingdom rather than a thriving undersea metropolis, and he actually talked to sea creatures in their own "languages" instead of using telepathy. Additionally, Aquaman had no civilian name, and none of his supporting characters like Aqualad, Mera, or even his most famous villains, Black Manta and Ocean Master, appeared until many years later. Aquaman was also a more violent character in his early appearances, killing Nazi villains with little issue. The differences were pronounced enough that, in 2022, DC officially declared that the Golden Age Aquaman was actually a separate character from Arthur Curry in the New Golden Age one-shot.
Green Arrow originally had a different costume, brown hair, and no goatee. He also had a more boring personality, lacking the inclination towards social justice that would define the character starting in The Bronze Age of Comic Books. Additionally, like many Golden Age heroes, he had no qualms about killing bad guys.
While Neil Gaiman's The Sandman (1989) has always taken place in The DCU, early issues were much less shy about depicting the title character interacting with other DC characters. Hell, the first story arc features Dr. Destiny as the Big Bad, as well as John Constantine, Mr. Miracle, Martian Manhunter, Etrigan, and The Scarecrow in supporting roles and/or cameos. For the bulk of the series, the Endless and associated characters more or less exist in their own continuity. In addition, while the comic was always very dark, the first story arc was closer to a straight-up horror comic.
Similarly, the Martian Manhunter, having first appeared at the start of The Silver Age of Comic Books, was more like a detective than a superhero in his initial appearances and only became a superhero when superheroes started to get popular again.
Also, there was absolutely no indication whatsoever that Mars was a dead planet or that J'onn was the Last of His Kind. Many of the tales of the time feature either Martian technology or the appearance of other Martian characters.
Upon reading the firstSilver AgeGreen Lantern story, you get the impression that the Green Lantern Corps (referred to only as space-patrolmen in the story) do not generally call themselves Green Lanterns — in fact, "Green Lantern" was only an alias Hal adopts for himself.
While John Stewart is primarily known for being a very serious, straight-laced military veteran, that wasn't always the case. In his early appearances, John was far livelier and less uptight, and had a clear rebellious streak. In fact, John's lack of respect for authority figures caused him to clash with Hal on more than one occasion. The modern characterization of John as a stoic former Marine wasn't introduced until the Justice League TV series.
Likewise, Guy Gardner was actually a perfectly nice, normal dude when he first showed up. His exaggerated Jerkass tendencies only really came into play after he recovered from his coma and became a full time Green Lantern during Crisis on Infinite Earths.
The Golden Age Green Lantern, Alan Scott, is quite different from the more famous Silver Age version. He was based out of Gotham, explicitly used magic, and had a weakness to wood. Also, the concept of there being a large cadre of Green Lanterns overseen by the Guardians of the Universe was introduced with the Silver Age version; Scott had no knowledge of or contact with the Guardians or with any other Green Lanterns.
Deadshot made his first appearance as a villain in a 1950 Batman story wearing an unrecognisable suit and having none of his later attributes. It wasn't until Steve Englehart's run on Detective Comics that he was given the design and personality that would make him the face of the Suicide Squad.
When Kingdom Come was released, Iris West II, the daughter of Wally West (the third Flash) was identified as being the fourth Flash in the script and concept art of the series, despite having a costume more in line with a Kid Flash (though she seemingly was an adult). Her follow-up appearances in the brief arc in Titans and The Kingdom instead identified her as Kid Flash. In fact, The Kingdom makes it a character point that Iris desperately wants to get her father's blessing to be the Flash.
Doomsday had quite a few things about him in his first appearance during The Death of Superman that made him the Trope Namer for Generic Doomsday Villain. When he first appeared, he was nothing but a rampaging brute as powerful as Superman but lacking any sort of morals (his Establishing Character Moment was letting a little bird land on his hand, then casually crush it and laughing). The full extent of his powers and even his origin wouldn't be expanded on until Superman/Doomsday: Hunter/Prey.
In the team's first adventure, the line-up only consisted of Robin, Kid Flash, and Aqualad. Wonder Girl was added to team in their second appearance, but Speedy wouldn't join the group until Teen Titans #19. The more notable additions like Cyborg, Beast Boy, Raven, and Starfire wouldn't be added to the team until the New Teen Titans relaunch many years later.
In the early issues, the Teen Titans had a hotline where they could be reached by teenagers from across the country, and their adventures often involved helping out normal kids.
In issue #25, Lilith Clay's first appearance, some perplexing dialogue (such as an announcer referring to her as "The enigma of the East" and Speedy calling her "A dish of sukiyaki!") would seem to suggest she's of Asian descent or possibly mixed race, something that was never followed up on.
In early stories, Beast Boy's animal forms would actually have the normal colors of the animal he turned into, with only his head being green. This was changed to the entirety of his animal forms being colored green like his human form. He also wore a full face purple cowl, since he had a secret identity and couldn't disguise being green in everyday life.
The earlier issues of the original run had the Chief claim that Cliff Steele's robotic body still allowed him to feel all five senses. Later stories would have Cliff angst over how his robotic body has robbed him of being able to experience touch, taste and smell.
There wasn't originally an explanation given for why Larry Trainor's body was constantly bandaged. Seven issues after the team's debut, it was explained that the accident that made him Negative Man also made his body radioactive and that the bandages he wore were to protect others from his radiation.
The first two stories gave Cliff Steele's codename as Automaton rather than Robotman.
The original 1960s series made it clear several times that Cliff could die if his head was removed from his body, when later comics would establish that Cliff can survive decapitation and will be okay as long as his head can be attached to a new body afterwards.
Justice League International: The earlier issues are much more straightforward superhero stories, with just a bit more comic relief moments than comparable superteams. The amount of comedic moments gradually grows, and when they become Justice League International in #7, they already have entire issues dedicated to Sitcom plots. Also, the first year of the comic sees them ditching characters like Doctor Fate, Captain Marvel, and Black Canary, and acquiring the signature characters of the period, like Booster Gold, Rocket Red, Fire, and Ice.
The Crime Syndicate are well-known for being evil versions of the Justice League from a world where morality is reversed, but their original depiction in the Silver Age wasn't a completely straight Mirror Universe counterpart to the League, with their home universe Earth-3 merely being a world where they're the only existing super-powered beings in addition to being evil as well as certain major historical events being reversed (such as Britain fighting the Revolutionary War to gain independence from America, or President John Wilkes Booth being assassinated by Abraham Lincoln). No heroic equivalents to the standard DC Universe's villains were confirmed to exist until the first Annual of DC Comics Presents, where Earth-3's counterpart to Lex Luthor showed up to help the Earth-One and Earth-Two Supermen defeat their respective Luthors as well as Ultraman, having been encouraged by the presence of the two Supermen to finally stand up to Ultraman's villainy. Nearly every interpretation of the Crime Syndicate's universe since would make it clear that the universe is inhabited by villainous counterparts to the standard universe's heroes and, conversely, heroic counterparts to the regular reality's villains.
The earliest Kid Eternity stories in Hit Comics had Kid Eternity becoming the historical figures whose skills he needed rather than merely summoning them from the afterlife to aid him.
Silver Age Green Lantern story, you get the impression that the Green Lantern Corps (referred to only as space-patrolmen in the story) do not generally call themselves Green Lanterns — in fact, "Green Lantern" was only an alias Hal adopts for himself.