His claws. Are they like knives, in that they have an edge? Are they basically needles? Are they curved slightly, completely straight or so curved they might as well be hooks? Do they come out behind or between his knuckles?
His height. Canonically and originally, he was a pretty short dude. But then he eventually got taller and taller, and once Hugh Jackman became mainstream media's perception of him, it got much more common to make him tall, yet characters still referenced his short stature.
How scruffy he is. Is he just a little disheveled with slightly bushy eyebrows? Or is he a full-blown caveman?
Are the brow points on his cowl short and spiky or long and blade-like? And are his eyes blank and white while wearing the mask, or are his pupils visible?
Is he genuinely ugly or merely rugged? These days he's usually portrayed as attractive but rough around the edges, whereas in the 70s to 90s, Logan would look like a hobo sometimes and a full-blown villain other times.
His hair. The pointy "horns" are consistent, but aside from that, artists can't seem to decide if his hair is slicked back and styled or just a bushy, wild mane. In the 70s, he also had a very pronounced widow's peak.
What costume does he wear? Until recent times, Logan usually stuck to one suit, first the classic yellow and blue, then the brown, and then back again - however, for the past several years, he's had a tendency to show up in concurrent titles wearing either one, or a different outfit entirely, usually without any explanation as to why.
Storm is quite infamous for this in regards to both her general appearance and costume. When the X-Men returned to wearing super-hero uniforms, Storm went back to wearing her "the Twelve" outfit. However, practically every single artist who drew her interpreted her outfit differently; changing the color scheme from black/gold to black/white on a whim, giving her boots and making her costume stop at the thighs, etc.
Storm herself varies from being "somewhat Asian" to "heavily African". Her own physical appearance also varies, and this happens with a lot of black and non-white superheroes. Her skin tone varies greatly, as do her facial features.
Her white streak started as a pair of streaks on her temples, moved to the center of her head, and then to just her bangs. They can't make up their mind whether it's natural or bleached in, either. (In The Movie, it was a result of dealing with Magneto's machine). At least a few comics show Rogue as a little girl and she has the white streak even back then, so it’s likely natural.
Rogue’s hair colour is also debatable since it’s commonly classified as “brown with white streak” but other artists make it closer to “auburn with white streak”.
Early on (Avengers annual 10, her Dazzler issues), Rogue was often drawn as middle-aged, with graying hair at the temples and visible crow's feet around her eyes. When she pulled Heel–Face Turn to X-Men she was drawnmore and more attractive until she became a Ms. Fanservice.
Speaking of fanservice, how curvaceous is Rogue? In her early comics she certainly wasn’t, but by the time of 90s she invoked the Most Common Superpower for her sheer size. More modern comics have tried toning her bust and figure down while others bring it right back.
Similar to Spidey, it was hard to know if black or blue was the intended appearance of the X-Men day-one outfits. Later comics alternate. There's also at least once instance of "blue for full-fledged X-Men, black for rookies" when the day-one look was being used as training uniforms while all characters had their own costumes out in the field.
He's a Holocaust survivor, making him old by default, but an early incident where he was de-aged and re-aged has left his true appearance and visible age as a matter of debate. He boasted that the process returned him to his prime, but did re-aging him return him to his 70+ age or did bring back to being a young man in his thirties? Different artists will depict him as either a well-built old man with gray hair, a frail old man with gray hair, or a young, well-built man with gray hair. Considering he has naturally very pale hair only confuses the issue further. Though the hair color also varies between stories, from light blond to gray to pure white to a plain, very unremarkable brown.
Like his contemporary Magneto, Charles is old, but many artists have gone back and forth over how exactly aged he is. The common depiction of Charles he has prominent eyebrows and a lined face (not to mention his iconic baldness) but some artists such as Jim Lee make Charles more handsome while others make him look more weird. His brows still remain consistent though.
He arguably owes his current status quo to this phenomenon. For his first handful of appearances, he turned into a snowman, but very early on (X-Men #8) he became able to turn into ice. And, indeed, the very first time Jack Kirby drew him as ice, he was obviously solid, transparent ice — Angel even compares his powers to the Invisible Girl. Somewhere down the road, it was decided that he just coated himself in ice during combat. His inclusion on Spider-Man and His Amazing Friends led to a generation of kids that knew what he looked like but didn't really know anything else about him and it became the common idea that he actually turned into ice (which is the most logical way to interpret his powers going on appearance alone). Eventually you started getting artists once again drawing him as transparent when in "iced up" form. Eventually turning into ice was made officially part of his power spread. Now, not only does he turn completely into ice, but no two artists can agree whether he looks the same as he used to, or like a normal person made of ice, or like some spiky-headed monstrosity.
Of course, when you can convert your body to a substance that you can manipulate at will, who says you have to maintain a constant shape in your transformed state? The Age of Apocalypse version of Iceman (which is where the "ice body" concept first appeared) embodied this idea. As flighty as the mainstream Iceman is often depicted, it's a testament to his ability to concentrate (or, possibly, his subconscious desire to fit in) that his shape is not in as constant flux as the AoA version.
She is usually small-busted, but damnit if Frank Cho can't draw women as anything but hyper-voluptous, hourglass-shaped glamour models.
While Alan Davis, whose depictions of Kitty in Excalibur are considered a definitive version, managed to strike a nice balance in drawing her in realistic proportions for a developing teenager (14-15 at that point), while depicting her older teammates Phoenix and Meggan as more curvy and voluptuous. Lampshaded very early in the original Excalibur run, when Kitty tried to masquerade as Phoenix in the latter's very skintight costume.
There's also her age. It's not so bad these days, and it wasn't so bad right after her introduction, but for the middle years? Sometimes she looked 12, sometimes she looked 20, sometimes she looked shorter than Rachel and Meggan but still 20 and the writers clearly thought she was being made to actually look younger (leading to strange instances where what you've thought of as a grown woman for the past 10 issues reminds us she can't drive).
He traditionally has eyes with black scleras and red irises and pupils, but other colors turn up all the time.
His hair length changes frequently, sometimes it’s held high, other times its draped past his ears and touching his shoulders.
How handsome is he? When Gambit first showed up he looked creepy and weird, then Jim Lee made him a Hunk. Artists bounce back and forth between making Gambit have Perma-Stubble similar to Wolverine or clean cut.
Exactly how Japanese is she? To explain Betsy is a purple haired British woman who swapped bodies with Japanese assassin Kwannon, so she has Asian features... some of the time. Other artists have Pyslocke look Caucasian (she’s also played by American actress Olive Munn in X-Men: Apocalypse). Even more bafflingly Kwannon in Betsy’s body has a epicanthic fold seen in Asian people which is very confusing.
Her hair colour? Purple or black? It’s usually the former but more than a few artists pick the latter.
Pyslocke is the Trope Codifier for the Boobs-and-Butt Pose but like Rogue a lot artists have dialled her sexiness back to realistic proportions while others keep her curvaceous. Nowadays Pyslocke covers up in a full bodysuit.
His original appearance, chubby looking nerdy guy with oversized hands and feet? Or rugged nerdy guy with oversized hands and feet? All-New X-Men settles on a Adorkable look but his height still changes so that sometimes he’s only a head taller than Jean but other times he’s only a little shorter than his adult self.
How hairy is his original form? Some artists, including Jack Kirby himself have drawn young Beast with a prodigious amount of (not blue) hair on his hands and feet, whereas others don't give him any body hair at all.
Notably "Cat Beast" (the version of Hank that is cat-like) has a tendency to look like a different animal depending on who draws him. He's been a grizzly bear, a lion or a baboon, and sometimes looks closer to the older version, "Ape Beast". Grant Morrison (likely to get the artists off the hook) unlocked this Pandora's box when he introduced the lion look as Beast's "secondary mutation," and he's been morphologically unstable ever since.
Does Hank have pupils? Or featureless eyes like Nightcrawler?
What do his feet look like? Are they merely blue and oversized, or do they look like big hands, like in the movies?
How furry is he? It's generally accepted that Beast is covered in blue fur, but most artists can't be bothered drawing detailed fur every time Hank shows up in a panel, so most of the time, he just looks like he has blue skin with some long tufts of blue hair on parts of his body.
Does he wear some kind of actual outfit, or is he literally running around in a pair of briefs and a belt? Due to his unique body type and since he's covered in fur, many artists don't bother giving Beast an outfit, since his appearance is distinctive enough. If he does wear an outfit, it's probably one with cut-off sleeves and shorts, though sometimes, he's actually worn a full bodysuit. That said, his original, more human-looking form is always shown wearing some sort of uniform.
Marrow:
Also of the X-Men, has had wild variations in skin color, from extremely pale to olive complexioned to slight purple tint. Her hair has seen similar difficulties; Pink? Red? Light brown? Purple?
Marrow's physical deformities are depicted entirely differently from artist to artist also - does she have bony ridges around her face or actual bones sticking out at odd angles from her forehead and shoulders, etc.? At least twice there's been an in-story explanation for this (her powers get altered) but at one point she was appearing in two different X-Men books that were supposedly taking place more-or-less simultaneously with appearances so different you could not have guessed it was the same character except through dialogue and process of elimination.
Much like Logan her height tends to vary. Sometimes she's roughly his height or taller, sometimes she's shorter. Her build is also inconsistent; at times she's depicted as very svelte and petite, others more muscular or athletic (though never to Amazonian Beauty levels). Her bust size varies almost as much as her overall build, with the artist for "Mercury Falling" in particular giving her quite an impressive rack.
Some artists draw her to resemble the teenager she's supposed to be, but others make her look much older, into her 20s. This has practically become canon since she assumed the Wolverine title in 2015, with both art and scripts treating her as though she's at least in her early 20s. That means she's out-aged multiple mutant peer groups, and the pragmatic definition for her age currently (ca 2018) is "as old as possible without turning her dating Angel into a statutory cradle-robbing situation."
Although canonically green, in her first appearance in NYX, and initially in New X-Men, her eyes were brown. Throughout the first volume of All-New X-Men they were blue like Logan's. Canonically and in most of her appearances they're green.
Abyss's appearance has widely varied within the same story arc. Made of wispy thread things vs. Nightcrawler-like furriness.
Astonishing X-Men member Hisako Ichiki (a.k.a. Armor) has the ability to generate Psionic Body Armor, the shape of which differs from artist to artist. Where John Cassaday would draw it as shaped like Samurai armor, other artists range between that and ginormous bubble suits, and this isn't even going into the color of it.
Hisako also suffers from this regarding how old she looks. Cassaday originally drew her looking like she's in her mid teens, but later artists make her look anywhere from a 9 to a 19 year old.
The member Sofia Mantega (Wind Dancer) is Venezuelan but she's drawn in a variety of ways, ranging from fairly dark skinned with South American features to Ambiguously Brown to very Caucasian-looking with blonde hair. Most commonly she appears somewhere in between.
Hepzibah, a member of the Starjammers from the X-Men setting, was an anthropmorphic skunk complete with pheromone powers, then someone apparently thought she'd look nicer as, or had a thing for, white furred elves with tails, leaving a character that's has "In Name Only" in common with the original. It doesn't even have the same quirky speech pattern!
Monet St. Croix constantly goes back & forth. The official Marvel site did an article celebrating its black heroes, and Monet is included in examples. Her father is Afro-French◊ and her mother was Algerian. But the artists & colorists can't seem to stay consistent with her race or complexion. Sometimes she looks black, ranging from caramel to dark chocolate in complexion. Other times, like her stint in X-Factor, she looks unambiguously like a white woman.
PAD said he got complaints from fans reading X-Factor, wondering why Monet was white all of a sudden. In response to that, they tried to gradually make her darker again. But even with that, she was still very light, with Word of God saying they still got complaints.
Hope Summers also constantly changes her look. Does she look like a little girl that has barely reached age 12? Or like a teenager? Sometimes she even looks like a red-haired Emma Frost. This stood out the most in Avengers vs. X-Men, where it changed nearly every damn issue.
, sometime he looks like a bear
