Original Superman was an outlaw hero not above dispensing rough justice in his earliest appearances. It is a little shocking for modern readers to see the character who would later become 'the Big Blue Boyscout' seize a torturer and fling him to his death. Likewise, given the international sensibilities that have long been part of the character's defining traits, it's quite jarring to see some of the World War II propaganda where we're told "Superman Says: 'You can slap a Jap with war bonds and stamps!'" among other political incorrectness.
DC later retconned these early out-of-character moments as being the Superman and Batman from Earth-2, so they're technically separate characters from the "main" Superman and Batman. Shows that they were aware just how much characterization had marched on in all those years.
John Byrne acknowledged this in Superman & Batman: Generations as part of the characters' evolution over time. In 1939, Batman takes one of the Ultra-Humanite's goons on top of a giant planet model to interrogate him and lets him fall when the mook doesn't give any useful information. When he's caught by Superman, Bat-Man remarks "If I'd known you were there to catch him, I wouldn't have let him fall", and Superman responds "If I'd known you'd let him fall, I wouldn't have caught him." They resume the interrogation and, when the mook still refuses to talk, Superman makes as if to throw him off the sphere once more, at which point he finally sings.
The New 52 has also characterized young Superman similarly as his early incarnation.
Modern interpretations of Hal Jordan have him as something of a womanizer and conservative, but in his earliest comics, he was liberal to an extreme given the late 50s. This would change in his crossovers with Green Arrow, who (as a Robin Hood Expy) defaulted to being more liberal.
It's kind of a shock to see how Guy Gardner was originally a pretty mellow character before (in a convoluted series of events) he received brain damage that manifested itself in the form of the arrogant, violent, unstable, and often childish personality that the character is best known for. DC has since tried to sweep this under the rug by retconning the brain damage thing altogether. Some flashbacks imply that Guy has always been, as he puts it, "the crazy one." Likewise, those who are most familiar with the mostly-stoic Marine in the DCAU version of John Stewart, which has since been incorporated into the comics, may be surprised at how, well, funky he is in his early comics appearances.
In The Silver Age of Comic Books, Sinestro was very generic and banal in his evil; his conquest of his planet (seen in flashback) involved him sitting on a throne, demanding tribute in the form of money and jewels in exchange for his services as protector and killing anyone who called out on what a selfish lout he was. It wasn't until 1991's Emerald Dawn II, that Sinestro's past was completely reworked to make him a Adolf Hitler-esque control-freak who conquered his homeworld "to protect it" as the driving force behind the evil he did.
Deathstroke was far more of a dirtbag in his earliest appearances, in sharp contrast to his eventual Anti-Villain status. One of his earliest appearances had him kidnapping Cyborg's love interest in order to lure the Teen Titans into a trap, something that would seem at odds with his Never Hurt an Innocent guidelines that would be established in later issues, and recreate the Doom Patrol's way of dying - a traumatic event for Changeling - For the Evulz. This can be explained by the creative differences between George Perez, who saw Slade as a ruthless villain whose "code" was only a personal justification, and Marv Wolfman, who did not see him as a villain but as an honorable victim of circumstance. Thus when Perez left the title, Deathstroke's more noble characterization took hold. However, Christopher Priest eventually restored Deathstroke to his original characterization in Deathstroke (Rebirth).
In the original comics, Plastic Man was no less serious than any other superhero at the time - he was the Only Sane Man of his world, which was filled with wacky, cartoony characters. In fact, even his powers were amusing, which made for an enormous contrast with his early, comically serious personality. Over the years, Plas received a much more light-hearted, jokester disposition, being mostly comic relief and Fun Personified in his modern portrayals, such as the one in Batman: The Brave and the Bold.
When she was first introduced in Legion of Super-Heroes, Monstress was basically a female Ben Grimm, in both speech patterns and attitude. Once she joined the team, the writers decided it was more interesting if this hulking monster was an upbeat fashionista who called everyone "dear" or "sweetie".
Green Arrow was originally and for a long time after his introduction mild mannered and apolitical - essentially a better adjusted equivalent to Batman, at a time when even Batman was more prone to smiling than his current incarnation. His left wing politics and spikier personality generally only became part of his character in the late Sixties, nearly three decades after the character's introduction.
Terra as originally written in The Judas Contract has become this over the years. In the original Teen Titans comics from the 1980s she was canonically "psychopathic" and "evil". She had no ulterior motive for being The Mole or hating the Titans, besides just being cruel and hating "goody-two-shoes" people. Word of God is that Terra's Age-Gap Romance with Deathstroke was there to make her seem even more gross. Terra's original interpretation, however, has caused a lot of Values Dissonance, especially with people criticizing the Slut-Shaming factor to her relationship with Deathstroke and noting how Terra is vilified while Deathstroke often gets treated far more positively. As a result, since the 2000s DC has made an emphasis to portray her as a less one-dimensional character, being a troubled youth who fell in love with an older man who ended up emotionally manipulating her (or even, in some comics, drugging her). She's still evil For the Evulz, but she's a more morally ambiguous character.
Amanda Waller was originally a tough, intimidating woman who also had a strong moral code. Indeed, much of her meanness was just an act, and she was willing to go against orders for the sake of her Suicide Squad. When she believed that the U.S. government had been corrupted, she took the Squad and went rogue. These days, however, most readers know her as a sociopathic manipulator who believes in My Country, Right or Wrong and throws away the lives of the Squad at the drop of a hat.
Wonder Woman was the only one of the characters who would go on to become DC's big three to start out with a no kill rule, and in the The Dark Age of Comic Books became the only one of the three not to have a strict no killing rule.
Trigon, in his first appearance in the first New Teen Titans story arc, was somewhat different from how he'd later be portrayed. Here he was presented as more of a Galactic Conqueror who happened to be a demon, ruling an interstellar empire from his capital on an actual planet and commanding mundane armies, and when the Titans faced him directly he came across as a powerful foe, but not insurmountably so. From The Terror of Trigon onward, he'd be presented explicitly as DC's premier Satanic Archetype, a Sealed Evil in a Can whose escape onto Earth immediately triggered Hell on Earth, and who is a Physical God who is so powerful as to be entirely undefeatable by mundane means. This depiction has been pretty consistent ever since.
