Healing Factor characters Wolverine, Sabretooth and Deadpool are depicted as sustaining injuries over a length of time in early appearances, compared to later appearances where healing appears near instant. Sabretooth's face was injured in The Spectacular Spider-Man (vol. 1) #116 and still scarred by his next appearance in #119. In an X-Force appearance, Deadpool complains his broken jaw left him hospitalized for a few weeks. One can assume characters had weaker healing factors in the past, but newer stories set in the past still depict a near instant healing factor.
One early issue implies that Charles has unrequited romantic feelings towards Jean, which he keeps quiet due to The Chains of Commanding and angst over being a Genius Cripple. Later runs would make Xavier old enough to have been Jean’s father, and to have recruited her into the X-Men as a child, which makes this notably odd.
In the first few issues of X-Men, mutants weren't hated and feared, the X-Men were treated as celebrities (the second issue has Angel meet a bunch of fangirls), they had an official government liaison (Fred Duncan), Jean Grey had actually been recommended to attend the Xavier school by what was implied to be a US intelligence agency, Beast was Dumb Muscle, Iceman was a Jerkass who would attack the rest of the team for fun, and Jean Grey's telekinesis was for some reason called teleportation, even in the second issue, in which they fought an actual teleporter. Xavier didn't hesitate to hit people who knew too much with Laser-Guided Amnesia, either. Also, Magneto, in contrast to his contemporary portrayal as a Well-Intentioned Extremist, was a straightforward bad guy without any redeeming features. He also had psychic powers that were nearly a match for Charles Xavier's, including Astral Projection.
Speaking of Beast, after leaving the X-Men, he briefly starred in Amazing Adventures, where he first gained his furry, ape-like form. However, the first issue was significantly Darker and Edgier, likely influenced by the popularity of horror comics in the 70s, and had a major subplot about Beast possibly losing his humanity and becoming a feral monster. This would seem quite odd to modern readers, since Beast maintaining his immense intelligence and friendly personality in spite of his monstrous appearance is probably one of his most iconic character traits. He also had a Healing Factor during these stories, something that was dropped by later writers. Additionally, back in the Silver Age, Beast was generally written as more of a stereotypically uptight nerd (save for the very early issues mentioned above, where Stan Lee wrote him as a dumb brute). Hank's jokier, more affable characterization wasn't introduced until Steve Englehart made the character a member of The Avengers during The '70s, and his sudden personality shift was actually notable enough that other heroes even commented on it in-story. Oh, and he was gray at first. The "Beast becoming feral" storyline was revisited in the first series of What If?.
For the first few years, all of the X-Men, bar Iceman, wore matching uniforms in dark blue and bright yellow, complete with cowls. They only varied slightly to accommodate their respective powers, i.e Cyclops had his visor, Angel had his wings and Beast usually skipped the gloves and boots. The uniforms were unisex too, Jean Grey as Marvel Girl had pretty much the exact same outfit as her male counterparts, although her hair remained visible.
Iceman himself started out looking like a walking snowman that ran around in a pair of boots. It took several issues until he started looking like he was either made out of ice, or coated in it.
In X-Men #1, Magneto can manipulate the magnetic field around objects, meaning he can move any object whether or not it contains magnetic metal; in other early stories, he seemed to be able to do anything as long as he was said to be doing it "magnetically", leading to amusing panels where Magneto seems to be able to control "animal magnetism" or can manipulate people thanks to his "magnetic personality". Professor X introduces a training machine instead of the Danger Room. In X-Men #2, Professor X is called "Dr. X". In X-Men #5, The X-Men have now finished their training at Xavier's school. In X-Men #6, The X-Men's cook is mentioned, and never seen again. Professor X and Magneto both have the ability to project mental images to contact Namor. In X-Men #7, The X-Men have now graduated from Xavier's school. In X-Men #8, Cyclops is called Sommers. The Angel is called Bobby. (Mind you, those two things seem like errors; in issue 1-7, both heroes have their familiar names, though in the first, Scott was called "Slim," which is now officially a nickname.) In X-Men #9, Bavaria and Bulgaria are the same location. In X-Men #33, The Juggernaut steals Professor X's mental powers. This is not brought up again.
While modern depictions of Cerebro tend to operate on the idea that only telepaths can operate it (save for special exemptions, such as when Kitty Pryde was able to modify the computer so that she could use it), that was not originally the case. In fact, Reed Richards once built his own facsimile of Cerebro, and was able to use it to track Karma in her first appearance.
Like Magneto, Emma Frost was a straight up villain in her early appearances, rather than the morally gray character she became in The '90s. Readers familiar with her Mama Bear tendencies and her love of children would probably be shocked to see the early New Mutants storyline where Emma kidnapped Kitty Pryde and tried to forcibly brainwash the young mutant into joining the Hellions. In fact, this would seem especially odd given the later X-Force issue where Emma told Warpath that she approved of his decision to leave the Hellions, and that she never would've forced any of her students to stay with her against their will.
Before he was Wolverine's Arch-Nemesis and one of the X-Men's most iconic baddies, Sabretooth started off as an Iron Fist villain. He was partnered with Constrictor in his second appearance (in which he was already much less efficient than in his first one), there were no hints at his mysterious backstory or connection to Wolverine, and he didn't have a healing factor either. He was also much less of a threat when he wasn't written by Chris Claremont, getting his ass handed to him by characters like Black Cat. It wasn't until the 1986 "Mutant Massacre" crossover (Sabretooth was introduced in 1977) that he began to become the villain we know today. It was the first story to have him fight Wolverine on panel, as well as the first one to reference their mysterious shared past. He was also played up as a much more powerful and intimidating character than he'd been previously, and was finally confirmed to possess a healing factor like Logan's. A possible explanation for the differences in portrayal is that Chris Claremont actually had plans to reveal that all of Sabretooth's early appearances (except for the very first one) were actually clones created by Nathaniel Essex. This would have, coinkydink of coinkydinks, left the only appearances of the "real" Sabretooth to be the ones Claremont wrote.
X-23, in her early appearances in Uncanny X-Men, was shown as mocking Psylocke's mannerisms, something that went against her usual stoic and loner attitudes.
Cable, being a character whose backstory was made up as things went along, has some instances:
He started out using a lot of strange idioms (notFuture Slang) which suggested that he spoke with a British accent.
The nature of his cyborg arm changed. Originally, it was very clearly meant to be a normal appendage, albeit with weapons and the like, as Cable is seen fixing and modifying it in his early appearances, and Forge repairs it for him without incident. Later on, it was established to be the techno-organic virus consuming his arm, and his telekinesis was what was holding it back — it naturally repairs itself and he cannot actually modify it or anything beyond making tentacles some out of it sometimes.
Early appearances heavily implied that he was formerly a soldier for the U.S. government, with the government being keenly aware of him and his service record. This doesn't line up with later revelations that he only returned to the past as a 50-something and ran unsanctioned mercenary groups for the most part.
His early interactions with Cyclops (according to his later backstory, knowing full well he was talking to his own father) show no sign of familiarity, to the point that Cable even addresses Scott as "Summers". This contrasts markedly with his sister Rachel's very emotional reactions to Scott and Jean in her early appearances after arriving in the present.
Quicksilver and the Scarlet Witch started their lives out as villains, part of Magneto's Brotherhood of Evil Mutants. They quickly left Magneto's employ and became members of the Avengers, which would more or less be their status quo for the next few decades.
When Gambit first appeared in Uncanny X-Men #266, he charged objects with green bolts from his eyes. Later appearances went with the much more intuitive idea to have him use his hands.
Mutant Massacre and The Fall of the Mutants were the earliest Bat Family Crossover events, not just for the X-Men but for comics as a whole and it shows. Both stories had a strange way of doing things unlike how things would happen in later events. For instance, in the Mutant Massacre, one could read, say, Uncanny X-Men to see what was going on, but if you wanted to know why X-Factor briefly showed up or where the New Mutants disappeared to, you could turn to those books to find out. Fall went with a strange Three Lines, Some Waiting take where events from one title are hinted at in another, but hold no real weight until the end. Inferno would be the Trope Codifier for just having the entire story being told consecutively through each major title.
Like the Avengers, the X-Men did not initially have their own Cool Plane, instead relying on their X-Copter (and later, an appropriated Sentinel craft) and civilian vehicles like cars, airliners and even cruise ships to travel. The team would not get their iconic Blackbird until the Claremont run.
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