In Arrow, the toned down 'no fantastical powers' rule results in this for some characters, by turning them into Badass Normal characters instead.
Harbinger/Lyla Michaels, in the comics, is a super-powered ally of the Monitor and joined the Amazons; in the series she's a former Afghan War vet-turned ARGUS agent and leader of the Suicide Squad, and Diggle's ex wife who he reunites with. Subverted in Season 8, where she's revealed to have been working with the Monitor the whole time.
Shrapnel, in the comics, is a walking pile of scrap metal. In the series, he's a mad bomber.
Laurel is the Black Canary in the comics, one of the best fighters in the DCU. Here, she's an OK fighter at best who can take on a thug or two, but is easily over powered by stronger fighters. To rub salt in the wounds, she's anticlimactically killed off by Damien Darhk before she was starting to become a credible Action Girl.
Sara, the show's Canary, also lacks her trademark Canary Cry. Instead, she has a sonic-generating device that has the same effect, so its more of a subversion.
Laurel's Earth 2 double in The Flash (2014) spin-off (known as Black Siren) does have the same ability, being a meta-human.
Kate Spencer is Manhunter in the comics. In-series, she is Starling City's District Attorney.
Count Vertigo, in the comics, is a super-villain who is well trained in combat and has the ability to disrupt and disorient opponents using the "Vertigo Effect" from which he takes his name. In the series he becomes The Count, an intelligent and influential but non-powered drug lord who manufactures and sells a narcotic called vertigo which induces a disorienting effect in users.
In the Doctor Strange pilot, Clea goes from being a powerful sorceress who can hold her own alongside Strange to a normal human damsel he has to rescue from the villains.
Watson again in Elementary, who has no combat abilities, "just" intelligence. She does get a level in badass combat-wise as the series goes, but 1) it's with stick-fighting/sword skills rather than firearms and 2) by the time she gets these skills she has semi-retired from being Sherlock's sidekick and thus on many plots she is somewhere else doing investigations of her own while Sherlock is having trouble.
In this version, despite his power, the titular character seems totally unable to beat an opponent without the support of his team (the majority of the time). Justified since not only is Barry still relatively inexperienced, he has a bad habit of rushing in without thinking as well as over relying on his super speed, otherwise he'd function as a Story-Breaker Power.
Jay Garrick, the first Flash, was a memorable hero who influenced the Justice League in the comics. However, the show outright states that he was afraid of his Arch-Enemy Zoom, and spent every moment trying to run away from the villain because he feared for his life. Flashbacks show that Zoom effortlessly defeated Jay in a one-on-one fight and was about to kill him before the two were sucked into a portal into Barry's universe. Later it is revealed that this Jay was actually Zoom's time remnant, deliberately created to instill false hope into people before crushing it as Zoom. Zoom eventually kills him off, and the real Jay Garrick is revealed to be a Flash from yet another world and a double of Barry's dad. He's also implied to have been a soldier once, restoring his badass status.
Zoom himself is also an example, due to the difference in his powers. While the comic version is a Time Master who is essentially invisible, can create sonic booms by snapping his fingers, and creates rifts in time just by being present, the TV version has standard Super Speed powers, though they were augmented by Velocity 9. In addition, this Zoom lacks his comic counterpart's mastery of psychology, though he makes up for it by being much better at long-term planning. However, it's entirely possible that this will end up being a subversion, since the Zoom shown here is the Earth-2 Hunter Zolomon, as opposed to his Earth-1 counterpart.
Firestorm in the comic has to ability to manipulate matter, but here is simply a sort of Human Torch. The transmutation ability starts to manifest near the end of the Legends of Tomorrow spin-off (although it's a different Firestorm due to Ronnie's death and his replacement by Jackson as Stein's other half).
Hercules from Once Upon a Time. The Hercules from the myths was able to complete his twelve labors. This version is killed by Cerberus and sent to the Underworld. He also needs Snow White and Meg's help to defeat Cerberus, thereby finishing his labors and allowing him to move on.
Jonathan Strange & Mr Norrell: in the original book the titular Mr. Strange's most notable magical power is his mastery over geography, allowing him to move around mountains, forests and cities like pieces on a chessboard. In the miniseries, on the other hand, he is once asked to move a forest out of an army's way, but finds himself incapable of the task because the trees have wills of their own and resist being moved.
Preacher: In the show, Cassidy the vampire is portrayed as about a physical match with the Badass Normal Jesse. He's also knocked out by a Tap on the Head. In the comics, Cassidy has Super Strength and superhuman toughness. He can No-Sell a bullet to the brain, and Jesse has his hand shattered when he catches Cassidy's punch. Jesse's climactic fistfight with Cassidy is portrayed as something of a Did You Just Punch Out Cthulhu? moment for Jesse.
Clark Kent aka Superman gets this as he's far less powerful than the comic book version or even other live-action versions for that matter and he's explicitly weaker than Kara who has defeated him, though he's actually proud of her for it. In the comics, while they can be more or less equal Depending on the Writer, Clark is still capable of feats that Kara has never accomplished.
Mon-El of Daxam in the comics had powers on par with Superman except his Kryptonite Factor was lead, but this version is considerably weaker than the Kryptonians or Martians, having slightly lower Super Strength and Super Speed than Kara and being unable to use flight, heat vision or superbreath.
John Jones aka Martian Manhunter also seems to suffer from this in the first season, being subject to The Worf Effect and getting knocked out whenever the plot demands he be unavailable. His telepathy is also much less skillful as he can't erase specific memories without wiping a mind completely. Some of his other abilities from the comics, notably density shifting and martian vision, are absent here. Despite this he's still most likely World's Strongest Man.
Lord Moran in the Sherlock episode "The Empty Hearse". Colonel Moran in the original Sherlock Holmes stories is Moriarty's top assassin who nearly manages to kill Holmes. In the TV series, he's a corrupt politician with no violent abilities. Although the TV Moran is a very minor character, the show's real version of the Conan Doyle Moran turns up in the same season as part of a Composite Character, the Sherlock version of Mary Morstan.
Irene Adler in the original Holmes story "A Scandal in Bohemia" has the primary claim to fame that she is one of, if not the, only individuals to completely outfox Sherlock Holmes. Holmes considers her a Worthy Opponent by the end of the story (not hurt by her Hero Antagonist status), and she even causes him to reevaluate his opinion on women. In the Sherlock adaptation of the story, A Scandal in Belgravia, she loses to him multiple times, only briefly coming out against him, and eventually gets rescued by him multiple times. On top of that, she's also revealed to be working for Moriarty.
The Witcher (2019) gives this treatment to Eyck of Denesle in the episode "Rare Species" which is based of the book Sword of Destiny. In the book Eyck is a truly badass Knight in Shining Armor and dragon slaying monster hunter whom even Geralt considers a rival. Apart from saving Geralt and Yennefer from a avalanche, Eyck actually fights the golden dragon Villentretenmerth himself before getting knocked off his house by the dragon's tail and crippled. In the show Eyck of Denesle is a Prince Charmless and treated like a joke, the only monster he kills is a Hirikka, a creature Geralt says was harmless and would've left them alone if they fed it. Instead of facing the dragon, Eyck is unceremoniously killed while shitting in the woods by the Reavers.
From The Tick (2001) we get Fish Boy: lost prince of Atlantis. While we don't really see him in action in the cartoon we do know that he is at least classified as an actual superhero. In the live-action series he's downgraded to a milquetoast of a sidekick, who is constantly physically and emotionally abused by his hero, The Angler.
Tyreese, due to being introduced later than his comic counterpart and losing his Lancer position to Breakout Character Daryl as a result. In seasons 4 and 5 they reworked him into a sort of Token Good Teammate who disliked the brutal nature of the post apocalyptic world, and struggled with many moral dilemmas that weren't present in the comic such as forgiving Carol for killing Karen and refusing to kill Martin even after he'd held baby Judith hostage and forced Tyreese to walk out into a small herd of zombies. His Death In The Limelight episode was even centered around Tyreese accepting in his dying moments that he just wasn't mean enough to survive in the kill-or-be-killed setting.
In the comics, Dr.Denise Cloyd was a competent doctor in Alexandria and was able to outsmart Rick Grimes. In the show, she is a psychologist who was forced to step up as the new doctor of her safe zone, despite being incompetent at the position, and is completely timid around others. The first time she must perform surgery on a dying patient she fails after cracking under pressure.
The Saviors start out much weaker than their comicbook counterparts. For starters, they suffer from numerous defeats all throughout their debut season and are constantly mocked by Rick's people for underestimating their strengths before they actually had to take a level in badass in order to match up to their original characteristics. Though this trope was deliberately in play in an effort to give them character development as Alexandria's greatest enemy after suffering so many attacks instigated by their people.
Number One aka Luther while he still has Super Strength, lacks a Jet Pack thus he isn't a Flying Brick like in the comics and is quite less powerful in general. For example, at one point in the show a falling chandelier incapacitates him, while in the comic Luther has bounced back from getting punched through buildings and getting hit with massive explosions. Luther also doesnt have his fancy (if ridiculous) Ray Gun in show. Like Allison, Luther is closer to his comic version during the glimpse of the Bad Future in the Season 2 premiere, as he shrugs off a missile shot at his back by a Russian soldier, implying Luther has the potential to be stronger than he normally is.
Wonder Woman: In the comics, Etta Candy and the Holliday Girls are a frequent significant help to Wonder Woman and are more than able to handle themselves in dangerous situations. On the TV show, Etta was reduced to comic relief whose high point was berating a Nazi spy for stealing $1.50 from her. Seriously, that was it.
The titular Lucifer was introduced in Neil Gaiman's The Sandman as the second most powerful being in all creation, second only to the creator, someone who the eternal and undying Anthropomorphic Personification of Dreams and Storytelling feared such that entire issue was dedicated to Dream putting his affairs in order and saying goodbye to people due to the possibility that Lucifer might decide to destroy him. In the show he's an angel, an immortal, and a badass, but doesn't appear to be particularly special compared to the other angels that appear in the show.
Beast Boy can only transform into a tiger and a snake, unlike his comic counterpart, who can morph into any member of the animal kingdom he can think of.
Superboy lacks many of his powers from the comics, including flight and tactile telekinesis.
The Boys (2019): Homelander never gets the better of comic Stillwell, who is completely unfazed by him and becomes a Karma Houdini in the end.
The title group also never inject themselves with Compound V to get superpowers. As the creator explained, doing so would reduce it to another superhero slugfest, while keeping them as average humans put wits as the priority over strength - "We don't need powers to save the world. We just need each other."