The Adventures of Tintin shaped the old comic book scene as we knew it. Hergé was far from the first European comic strip artist, but he did combine elements from American comics (Speech Balloon, dynamic drawing styles) with lots of documentation, impeccable art work, page turning suspense, satire and compelling atmosphere. There is still an entire school of comic book artists (Ligne Claire) dedicated to imitating Herge. As a result it can come across as cliché to young modern readers who've read other adventure comics.
Most Underground Comics fall in this category as well (especially Robert Crumb). Back in the 1960s and 1970s, these comics were seen as edgy and subversive, dealing with topics most other comics didn't deal with, including sex, politics, swearing and drugs. Nowadays, in a time where even animated sitcoms like The Simpsons, South Park and Family Guy feature extreme violence, sex jokes and political topics, one might wonder what made these comics so special in the first place.
Air Pirate Funnies, the infamous Disney parody featuring Mickey and Minnie having sex to the point of lawsuit and today's Rule 34 in Fan Art, for irritated Disney fans.
Superman may very well be the flagship example of this trope on comic books; many may wonder what made him so unique, until finding out that this is due to Supes being the most groundbreaking and pioneering character within the costumed hero genre of comicbooks, inspiring and/or influencing almost all that came after (and even before).
Throw in Squadron Supreme as well. It actually beat Watchmen to the superhero deconstruction idea. Or Bill Willingham's series The Elementals, which came out a year before Watchmen and The Dark Knight Returns. Or even the 1953 MAD parody "Superduperman", which is basically just Superman acting like a jerk. Not very impressive after, like, every MAD parody since then, not to mention something like Watchmen. Of course, it's the ancestor of both - yes, it was an explicit inspiration for Watchmen too.
Some of the "What If?" aspects of The Dark Knight Returns can feel a bit stale today, as they've since been integrated into the Bat-mythos as official canon. Specifically:
Having Batman and Superman forced into a battle to the death was shocking in 1986, since the two had always been portrayed as the best of friends before that. Since then, the stark contrast between Superman's idealism and Batman's pragmatism has become an essential part of their character dynamic, and it's traditional to depict them as complete foils who have difficulty trusting one another.
The revelation that Jason Todd was killed before the events of the story held a lot more weight when the book first came out, since the character's death hadn't yet happened in the regular comics (A Death in the Family didn't come out until 1988), and the idea of Robin being Killed Off for Real was still unthinkable to most readers. Now that Jason has since died and been resurrected as a badass Anti-Hero with his own series, hearing Batman angsting over his death can cause some eye-rolling.
Though a minor point, the inclusion of a female Robin, Carrie Kelly, doesn't seem quite as unique and daring now that Stephanie Brown has since served a brief tenure as Robin in the main DC Universe.
The depiction of The Joker as a mass murderer (complete with the story casually slinging around triple-digit numbers as his supposed body count) with strong Foe Yay overtones toward Batman also originated with this story, as did Batman's internal angst over whether his Thou Shalt Not Kill code meant that he was responsible for every person the Joker has killed. All of these elements are largely taken for granted in any modern Joker story (granted, the Joker did kill people before The Dark Knight Returns, but the level of seriousness with which those stories - and Batman - took those murders bordered on Angst? What Angst?).
Being an Unbuilt Trope of the '90s Anti-Hero, Watchmen seemed especially poised to suffer this. The chances of getting lost in the myriad of imitators when the original wasn't even being played straight seems very likely (since all the imitators just zoom straight in for the most obvious traits while the Unbuilt TropeMaker is trying to make a point that didn't really exist before; ultimately while not having the same intentions, they result in looking similar to the untrained and unknowing eye).
The Authority was revolutionary for its violence and political themes. Today, it fits right in with most modern comics.
The Dark Phoenix Saga. Before Jean Grey's death was retconned a dozen different ways and the concept of "dead is not dead in comics" became a punchline for critics and comedians, a story where a main character becomes a morally grey antihero who sacrifices herself to save the lives of her team was virtually unknown in comics. It's become almost commonplace to kill off superheroes in "event" storylines these days (to the point that some comic fans take bets to see how long the character will stay dead). To today's average reader, Jean's death isn't all that shocking.
Deathlok. The comic book character debuted in 1974. In 1974, the idea of a man who has been turned into a cyborg and struggles to keep his humanity while fighting against those who transformed him was relatively original. Nowadays it's become a cliché, ripped off by Robocop and many other sources.
The Fantastic Four introduced the concepts that revolutionized the genre in the early 1960s. It was unimaginable for readers back then to have a superhero with a monstrous appearance like the Thing, or dysfunctional team dynamics (that became so popular, the FF looks normal in comparison with most other groups). That's not to mention the villains, which included a dangerous leader of a foreign country and a planet eater entity bound to destroy the universe. And they didn't have secret identities, which were a staple for all superheroes then (and are still common even today).
The Avengers #16, the iconic "The Old Order Changeth," story, completely upended the series' status quo by having almost the entire team resign, leaving Captain America to lead a new team of Avengers that consisted of lesser known characters like Hawkeye, Scarlet Witch and Quicksilver. Since then, the idea of superhero rosters drastically changing has pretty much become a trope in its own right, but back then, the idea of getting rid of most of a book's A-list characters to focus on a group of second-stringers was unheard of. Similar team books like Fantastic Four or Justice League of America generally had static casts, and while new members did sometimes join, the core casts usually stayed the same.
Legion of Super-Heroes. The Great Darkness Saga is considered one of the all-time best Legion stories. The villain is Darkseid - a plot element that seems trite nowadays because of Darkseid's overexposure. But the story is from 1982, when that was a new idea - back then, Darkseid was a very obscure character who showed up in a low-selling comic from a decade ago.
Spider-Man was a unique deconstruction of superheroes when he was first created. The idea of a superhero who was a normal teenager like the readers and who had a normal life hadn't been done before, along with the idea that superpowers not only couldn't solve your personal problems, but could make your life worse. Nowadays, this is nothing new.
"The Night Gwen Stacy Died". Today, especially with The Dark Age of Comic Books, it's not rare for characters to be killed off left and right, but back then, the thought killing off such a beloved and popular character was unfathomable. It's often credited with ushering comics into the Bronze Age.
The series, along with X-Men, also first popularized the idea of superheroes who weren't wholeheartedly embraced by the public, and often faced fear and suspicion from the people they were protecting. The idea soon spread to the rest of the Marvel Universe, and even to DC.
Both the X-Men and New Teen Titans were revolutionary for the time, and set the gold standard for angst, melodrama, and Soap Opera elements in superhero comics. Nowadays, it's quite hard to see what was so special about the books, as it seems like an unwritten rule that every superhero team book must contain a copious amount of love triangles and angst.
Back in the 1980s, Crisis on Infinite Earths and Secret Wars were big stuff. While characters had crossed over with each other before, earth- and universe-shattering perils so huge that not just one or two, but every single superhero (and villain!) within a given publisher's universe had to combine forces to defeat them was novel and exciting, completely unknown. Nowadays, the Crisis Crossover is a standard part of the superhero comic book publishing schedule, with at least one big event (sometimes more) happening every year.
The early comics of Jack Kirby and Joe Simon have a layout that was revolutionary at the time but is par for the course nowadays. Back then, comic books were very unimaginative in their layout. They looked as if they were reprint collections of newspaper comic strips even when they weren't, with one strip above another. The idea that you could do splash pages, or have one panel take up half a page, or use diagonal gutters and odd shaped panels... All these things were completely new at the time.
One minor example from the pages of Superman: Brainiac's name. To many non-fans, or just casual readers who like the character but find his name laughable, it can seem a bit ridiculous to name an ostensibly serious supervillain after a juvenile slang word for "genius". It can seem that way... unless you know that the character actually came first, and that the writers of Superman are credited with coining the word. The fact that the word is now part of the popular English lexicon, and that most people who've said it aren't even aware that it's a Superman reference, is just a testament to the comics' ubiquity.
"The Green Turtle" - An otherwise forgettable comic were it not for a few things: Back in the day, this comic was pushing it. How so? For one, Word of God states that, despite Executive Meddling, the Green Turtle is in fact Asian - his face was never shown and his skin tone somewhat exaggerated towards pink as a form of Writer Revolt. And a lot of people didn't realize it. It's hard to appreciate just how much this comic was pushing the limits of what was acceptable for the time.
In general, Golden Age and to a lesser extent Silver Age comic books are often difficult for modern fans to fully appreciate. Changes in computer and publishing technology have allowed for current comics to have very detailed and high-quality art, making the basic coloring and simpler line work of older stories less palatable; many innovations in composition and design had yet to take place, making them feel a lot more flat and bland; and the writing styles of older eras, when compared to the more natural and movie-like dialogue of today, were very wordy, melodramatic, and eccentric, and often perceived as juvenile. Add all that together, and it's quite common to see casual comic fans read through Action Comics #1 or Detective Comics #27 and wonder, "how did that manage to take off?!"