Many superhero comics. Superman was 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"); Batman and the Teenage Mutant Ninja Turtles both killed their adversaries, even Joker and Shredder.
During the early years, DC's different heroes very clearly took place in separate continuities - an issue of Batman from the 40s, for instance, had Dick Grayson get an autograph from Jerry Siegel, identified by name and explicitly noted as "the creator of Superman".
Early Timely (Marvel) Comics did the same. In the very first Captain America story, the Human Torch is mentioned as being "a hero from the comic books". About a year later, Cap and the Torch met in a crossover story.
Superman was probably the most notably different of these lacking many of his current powers, sporting a different (and often inconsistently illustrated) costume, killing bad guys, and being 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 2 era covers.
In part this is because Superman, Batman, and some of the others listed here were introduced before the genre was codified as they were the codifiers of many of its tropes. Their early adventures were the Early Installment Weirdness of the entire genre.
Superman wasn't always the well meaning hero he's known as today. In fact, he caused more harm then good in his earlier comments, and was unbelievably insane. Examples include 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.
The Brazilian distributor of Disney Comics released in 2000 a special celebrating the 50th anniversary of their Donald Duck magazine. The description of the first issue listed all that could be found weird: differences in language/spelling (apart from the 1950s version being quite formal, Portuguese underwent several orthographic reforms) and character names (biggest one was Goofy being called Dippy - though the story has him being called "a goofy guy"), unfinished stories (it was published across 3 issues as movie serials were popular in those days), only eight pages in color, and a trivia section filled with Values Dissonance (telling stories of animals suffering accidents, such as jiraffes being decapitated by telegraphic wires). 10 years later, the collection released to celebrate the 60th anniversary had its first issue come with a facsimile of Donald Duck #1, letting readers experience Early Installment Weirdness firsthand.
The early Sonic the Hedgehog comics were very comedic and often even stranger than what we have today. This was more due to following Adventures Of Sonic The Hedgehog and using a few video game stages in the early days. Sally was seen with varying colors of fur (red fur with blonde hair in her first appearance, pink fur with black hair up until issue 16) and Rotor was known as "Boomer" until about issue 6. Roboticization was shown differently with people with hypnotized eyes instead of out and out robots. The series wouldn't reach the SatAM levels until Dulcy showed up in issue 28 and a few characters will still seem odd looking back at them (for instance, look at Metal Sonic in issue 25 and look at him today).
Sonic The Comic was just as bad. For a while it was full of one-shots that barely related to each other, often with Off Model drawings. It took several issues for it to become plot-orientated, and even afterwards it took a while for it to drop the oneshot routine.
Stan Sakai has stated that he regrets having made Hikiji human.
The first few Hellboy stories are very odd to read in comparison to later installments, thanks to John Byrne's writing. Under Byrne, Hellboy uses a Private Eye Monologue to describe the story, tying him more closely into the Nineties Anti-Hero archetype. When Byrne left and Mike Mignola, the series' creator, took over dialogue in addition to plotting and art, this form of exposition vanished.
In addition, while the comic was always very dark, the first story arc was closer to a straight-up horror comic.
First two Nemesis the Warlock stories - Terror Tube and Killer Watt - refers to Torquemada as chief of Tube Police, while later he is referred to as "Grand master Of Termight". Nemesis spends both stories inside his ship, with no clue about his identity or appearance, saying nothing but his Catch PhraseCredo!, which he tends to use at odd moments. A lot implies it wasn't even established that Nemesis is even an alien back then, with narration referring to him as "not ordinary man" at best.
While people often note the various changes that came about when the Teenage Mutant Ninja Turtles changed from being merely a comic book to a multimedia franchise, the original Mirage comics were also subject to quite a bit of this. Most notable is the Frank Miller-esque narration, which was gone by the series' second issue; the idea that the turtles had grown to look like their present-day selves in the space of a year, which was ignored when the writers decided to write stories featuring younger versions of the characters; and the bit where Splinter expressly trained the turtles so they would kill The Shredder for him, which was glossed over with time. The Shredder himself was originally a Token Motivational Nemesis and Starter Villain who died in the end of issue one. He did kill Hamato Yoshi, yes, but he was pretty much the crook who shot Uncle Ben but with a cooler outfit.
Also, the Human Torch looked like a featureless yellow blob of flame, taking after his Golden Age incarnation.
As well as the Thing looking very lumpy and uneven.
Doctor Doom acquired the ability to switch minds with whomever he had eye contact with. This is an ability that he occasionally uses even in the newer comics, but it was an odd concept for the time. A later Retcon stated that only his infamous vanity keeps him from using the ability more often.
Ultimate Marvel Team-Up was done very early in the life of the Ultimate universe. As a result, a lot of characters like Hulk and Iron Man appeared before they were given headlining roles in The Ultimates and are noticeably different then what would come.
After his first appearance in Tales of Suspense #39, Iron Man spent the next few issues travelling back in time to visit Cleopatra, fighting a robot caveman built by aliens, preventing a race of people who lived inside the Earth from invading the surface world, and having several other wacky adventures. It wasn't until Tales of Suspense #45 (the first appearance of Pepper Potts, Happy Hogan, and supervillain Blizzard) that he started to edge more towards superhero territory.
Similarly, the Martian Manhunter, having first appeared at the start of the Silver Age Of Comics, was more like a detective than a superhero in his initial appearances and only became a superhero when superheroes started to get popular again.
Notably, Bruce Banner originally turned into the Incredible Hulk at night. He was also originally coloured grey, but this changed to green, as grey was hard to reproduce consistantly in the 1960's.
Of course, these two traits were notably brought back in years later in the 1980's where it was revealed that the Grey Hulk was another personality of Banner's.
There is a lot of other weird things that happen before Hulk hits his stride, him changing via machine, Hulk's intelligence level going up to near Banner and down to inhuman stupidity. The early issues also make the Hulk quite misanthropic (an extremely bad thing when coupled with Banner's brains) and Rick Jones was the Kid with the Leash who kept him from actually trying to end the world at least once. It takes them awhile before the character is associated with anger, around his first visit with the Inhumans where they establish that his strength increases with his rage.
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.
Knights Of The Dinner Table: The series started as a comic strip in the back of SHADIS gaming magazine and as such, the character were flat and Sara had not been added yet. The Knights were simple gamer stereotypes shifting to fit the gag of the strip (for example, Dave, Bob and Brian all knowing the stats for a monster whereas in later strips, only Brian had stats and charts memorized like that.)
This can be applied era to era in comics but especially the Silver Age which generally has the silliest and most over the top plots and really sticks out from the other eras with many of the most popular characters of today being invented or taking on their most recognizable forms in that era.
The original Archie Comics looked nothing like their more familiar, modern look. It had the more realistic style of most 1940s comics before becoming stylized like it is today. Archies looked more like Alfred E. Neuman than anything. The characters were also younger and it initially lacked the signature Betty and Veronica (Betty was part of the strip since day one, but Veronica did not show up until the fifth or sixth appearance).
The first three issues of Ninja High School were drawn in a somewhat different style from the rest of the series.
The "Locas" stories in Love And Rockets initially had sci-fi elements, but they were soon dropped and the series became very present-day real-life in nature.
Doctor Strange started out looking much older than his eventual appearance, with slanted eyes and vaguely Asian features, and was styled the "Master of Black Magic."
In the first few isues 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 liasion (Fred Duncan), Beast was Dumb Muscle, Iceman was a Jerk Ass (mission briefing would sometimes lead to him attacking the rest of the team) and Jean'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.
In early issues of The Beano pretty much all the comic's most iconic characters had yet to appear. The only strip in the first issue to survive into The Fifties was Lord Snooty. Early issues also included text stories and adventure strips unlike later ones which only featured humourous comic strips.
When Dennis the Menace first appeared in 1951 he lacked his red and black stripey jumper and instead wore a tie.
In the early strips of Mortadelo y Filemón, both worked alone as private detectives instead of being secret agents, Filemón was always angry and smoke a pipe, Mortadelo was even more stupid and clumsy, and there was barely any of the Slapstick that defines the series nowadays.
The series La parejita started in 1993 as "Emilia-o", the story of a 18-year-old couple who had to serve in the Spanish military service. Emilia, the girl, disguised herself as a man to stay with her boyfriend during that time instead of being sent to different destinations. After a year of military jokes, author Manel Fontdevila decided to put the couple back in the city and write a story about their day-to-day life, which he described as "putting the same actors in a different set".
On their very first appearance in "Johan and Peewit: The Flute with Six Holes", The Smurfs had five fingers on each hands, their village was on a rocky, barren landschape and their houses were different sizes and species of mushrooms.
In WITCH, Will was a major Butt Monkey and it seemed all she existed for, beyond being leader, was having something horrible happen to her. She lampshades this when her dormouse is ran over after she and her friends save her family's livelihood from her Jerk Ass, gambling-addicted father
Astérix the Gaul had an single issue Art Evolution (with plenty of Off Model drawings before Uderzo settled on the style he employed for years to come) and a plot unlike the ones that followed, in particular for having a relatively small role for Obelix. The first issues also have the lack of Dogmatix before his introduction in Asterix and the Banquet.
Superlópez began as a direct and shameless parody of Superman, mocking several Marvel Comics characters and plotlines. When the original writer Efepé left, it changed into a "regular dude thrown into a superhero role", focusing notably on López's life and work. From the 90's on, though, a decent rooster of villains had been added and the stories became more adventure-oriented, with several political jokes and references thrown everywhere.