Green Lantern's ring. The two most used descriptions for it are "the most powerful weapon in the universe" and "it can do anything you will it to". However, what this really translates to is "you can make glowy items with it". Any time a Green Lantern does something besides making glowy items with the ring that can, remember, do anything, other people react with shock, and it's generally a huge story point.
Later comics try to fix this, mostly because a glowy item may be the best solution for a problem.
Kyle Rayner's ring was explained to be different, that his ring could "create anything that he wills it to." This may have been to go back on the concept of "can do anything you will it to do, but you will only make glowing boxing gloves with it". Or possibly to highlight his background as an artist and thus will create giant mechs, video game characters, robots, and other fun things that were not glowing boxing gloves. (Except in that one instance.)
In Geoff Johns' Rebirth series, it's revealed that using the ring for anything requires a huge amount of stamina and willpower. When Green Arrow used it to make a glowy arrow he felt like a total wreck afterwards. When he asked Kyle if that's what using the ring feels like, Kyle answered "Every time". Using the ring for anything grander than a glowy item would probably leave the Lantern badly weakened. Since this particular trait hasn't been seen before or since, it comes off as an Informed Flaw. It also doesn't help that "willpower" and "imagination" aren't the same things.
This would also seem to be belied by the numerous times various Green Lanterns are depicted using the rings for completely unimportant things, like making a big hand just to point at something. You'd think if it was that difficult they'd save their strength.
This is partly The Artifact from the earlier days of the franchise, when Green Lantern rings really coulddo just about anything. Alan Scott in particular seemed to reveal new powers on every page. By the 80s, the average Lantern powerset was more or less standardized to "fly through space, shoot lasers, make green stuff", but the branding of "most powerful weapon in the universe" was too nice to drop.
Hal Jordan is usually described as "the greatest Green Lantern of all." Most of the time, though, he seems pretty average, and not much stronger (if at all) than the other human Lanterns, and quite a few Lanterns have accomplished things he's never even come close to. He also has some fairly consistent weak spots (most obviously, not-great creativity or intellect, when he has an Imagination-Based Superpower) that would probably hamper his ranking. It's even funnier if you go by his performances in the Silver Age, where he was almost notorious for getting knocked out on a routine basis.
The Ten-Eyed Man often places on lists of "lamest Batman villains" for his rather... odd... ability of having his eyes on his fingertips. Thing is, he wasn't intended as a joke; many characters talked up the awesomeness of his power and considered him a potentially unbeatable opponent. It goes without saying that a villain who can be defeated by tossing a sharp object (or any object, really) and shouting "Catch!" wasn't going to live up to that hype. He was literally once defeated using a pot of shrimp scampi sauce.
The comics repeatedly refer to the character of David Cain as "the greatest assassin on the planet". Yet Cain has never actually succeeded in an assassination he was hired for while on-panel. Cain did have one impressive achievement: murdering Vesper Fairchilde and framing Bruce Wayne for it (all off-panel). It took the Bat-Family a lot of time and effort to disentangle that mess. True, Cain had Lex Luthor backing him with the orchestration, but Luthor didn't know Bruce Wayne's secret identity, so Cain still gets credit for the planning. To top it off, Cain planted evidence deep enough that the police wouldn't find it, but Batman's proteges would, and the evidence made it look like Vesper was killed because she discovered Bruce is Batman, which really threw some doubt on the situation.
It's often stated that The Joker's greatest advantage is his insanity, which makes him utterly unpredictable to the point where it's impossible to guess what he'll do next, even for him. In practice, though, he seems to have no problem with preparing complex plans, and 90% of the time, his goals are something along the lines of "steal something," "kill a dude," or if he's feeling really creative, "try to drive Batman nuts," which is hardly unpredictable for a psychotic criminal. He also seems to be fully aware of his own actions, which casts doubt on the idea of him being insane at all.
Storm's leadership qualifications can fall short in practice. Everyone hails her as a legendary leader, despite her tenures tend to result in the worst setbacks in X-Men history. Before M-Day, more team members died (or appeared to have) than any other's. Lost Rachel? Who cares. Wolvie's MIA. He can take care of himself. Psylocke dead? Twice. Abandon the X-Men's mission to hide in the Outback? Sure why not? Use that time to take lethal action against foes like the Reavers and Marauders? Absolutely. Berating Wolverine when he does exactly the same thing? Priceless.
Professor Xavier is repeatedly stated to be the strongest telepath in the world, but you'd never know it since most every other telepath the X-Men have met have crushed Xavier in a mental battle rather easily. To further cement this trope, an attempt by Xavier to read a normal, unpowered soldier's mind was thwarted by said soldier thinking of porn.
X-Men villain Omega Red inevitably goes into great detail concerning his death spores and his energy draining death force something something. This is such a passive ability most of the time we have to take his word for it.
Mutants whose power includes energy manipulation, like Sebastian Shaw, will usually spend more time talking than using their actual powers.
The Neo, the next step in mutant evolution, so we're told repeatedly.
Tim Drake's Robin is supposed to be a brilliant leader on par with his predecessor Nightwing in the Teen Titans comics. Except that his team mostly does what they want, when they want. And they keep quitting because he and Wonder Girl are both assholes. Indeed, Robin's leadership is mostly shown only as him shouting "You, fight him! You, fight her! The rest of you, fight the rest of them! Go!"
Subverted in Young Justice where Tim/Robin displays amazing leadership almost all the time. This, of course, sadly means his character regressed as he aged and joined the Titans.
Megatron from The Transformers: All Hail Megatron claims to be an amazing tactical genius, but his abilities do not even come close to living up to the hype. It's noticed by the other Decepticons, too: when Starscream once again rebels, he's able to convince a lot of the other Decepticons to join him in fighting Megatron because of it, where as before he'd only ever been able to talk one or two warriors into joining him. This even got lampshaded in The Transformers: More Than Meets the Eye by a former Decepticon who's become a successful standup comedian: one of his most popular routines is about how little sense Megatron's big Six Phase Strategy made. A later speech by Starscream seemed to be a big Lampshade Hanging on the idea, characterizing Megatron as a charismatic nobody who ended up in the right place at the right time, and his ridiculous hype is basically him trying to cover that up.
Ironfist, of The Transformers: Last Stand of the Wreckers, is regularly noted to be a talented writer who is widely-read across the warfront. However, we see a snippet of one of his works in Bullets, and it practically veers into Stylistic Suck (it was a direct parody of longtime Transformers comics writer Simon Furman's writing style). Then again, he does apparently get a fair bit of ribbing about his style, and the tastes of an alien robot might simply be different compared to a human.
The Marvel villain Siena Blaze was said to be especially sociopathic because she kept on using her powers without reservation even though every time she used them there was a chance they'd accidentally destroy the world. Needless to say, this was never actually put into practice.
Justice League of America: ex-hero-turned-villain Triumph had numerous ones but unlike most examples, it wasn't for a lack of trying; he was quoting new powers in practically every panel he appeared in. His powers revolved around complete manipulation of the electromagnetic spectrum and among his stated-but-never-seen powers: generating a microwave pulse, perceiving radio/satellite signals and being able to kill Superman by sucking the solar energy from his body. He also claimed to have other powers the heroes "didn't even have names for" so his list of informed abilities was potentially endless.
Satirized in "The Authority" - Seth claims to have "over 1,000" different powers. Among them: Nuclear poop-vision, speed-squared, shame-o-vision, and rainbow breath. Seemingly none of the listed powers were worth using against the Authority, however.
Carlie Cooper. All those amazing qualities of hers are never shown, most likely because she hasn't been published long enough. Carlie is possibly an odd combination of Informed Ability and Composite Character. Those things that are supposed to make her great? Nerds Are Sexy, Beautiful All Along, falling for "Peter, not Spider-Man" and having a Dark and Troubled Past involving her dad? Deb Whitman, Gwen Stacy, and Mary-Jane Watson would like to have a word with you.
Gwen Stacy's science smarts. They were brought up in one issue during her original appearances, and never mentioned again. It's only in flashback issues that she's actually shown using science. They get played up in adaptations, seemingly to make her more unique or approachable, or in some cases, more useful.
Wolverine can sometimes fall into this category in regards to his martial arts training. He's said to be one of the most formidable fighters in the Marvel universe but 90% of his attacks involve simply jumping at his opponents and slashing him/her with his claws: a move anyone with two legs and claws can perform. He often lacks the finesse of other comic martial artists such as Captain America or Batman. It doesn't help that, due to his Healing Factor, he is much more likely to get shot, stabbed, and otherwise mutilated by common mooks. Sure, he gets better almost immediately but it can make him look less skilled than his peers who rarely even get touched by mooks.
Similarly, X-23 can fall under this trope sometimes, reverting to wildly thrashing and slashing with no apparent strategy, even though she has used expert precision in certain fights. Thankfully, this happens less often with her than with Logan.
An even worse offender than Logan is his son, Daken. The guy is supposed to be a highly-trained warrior, but from most of his appearances, the guy is repeatedly maimed and beaten, by far less skilled opponents, no less. His entire game plan can be summed up as: Attack! Attack! Attack!. In one of his appearances, he is very easily taken down by a group of Z-list villains that Norman Osborne all but lined up for him to defeat. Heck, The Punisher almost KILLED Daken (to get revenge on Daken for killing Frank once, in case you are wondering). And would have killed him too, were it not for Wolverine's intervention.
The Marvel villain Arcade is consistently described as a genius in mechanical engineering. However, nothing that he's built in his Murderworld (robots, deathtraps, ect...) appears to use any technology more advanced than what other villains may be able to purchase or obtain. He simply appears to be applying existing technology to his own purposes. Considering the fact that he's never defeated any superhero in his Murderworld suggests that his technology is quite subpar to that of other more high profile supervillains. He claims that he was once the best assassins in the business, and started building the Murderworlds because he was bored killing people in mundane ways. At least, that's what he claims. He hasn't been known to have killed anyone important in his career as an assassin, and not only does he not even have one win against a super-powered adversary to his credit, but even Courtney Ross, who was just a civilian (albeit one who can handle herself decently well), managed to survive his deathtraps. What's more, when his victims make it through the Murderworld and he's out of tricks, he usually runs for the exit as fast as he can, and if he can't make it, pretty much anyone can flatten him with one punch. If he was ever a true assassin at all, he would likely have been a pretty bad one.
With that in mind, however, he'd be a lot more competent if he avoided being in the same vicinity as the heroes and didn't toy with them. He always manages to at least capture his enemies before bringing them to Murderworld, and has been in the position to kill some of the world's greatest heroes multiple times. If he wasn't so obsessed with making a fair....ish game out of killing, he'd have murdered Wolverine, Spider-Man, the Thing and other A-List heroes decades ago.
Considering that Arcade's more recent Murderworld technology was courtesy of the efforts of Miss Coriander, Arcade's own aptitude with technology really comes into question. Once again, Miss Coriander's contributions aren't anything we haven't seen before.
Terra's creator, Marv Wolfman (who's always insisted that Terra was never anything butEvil All Along) later tried to rectify the relative lack of evil deeds to match the rhetoric by having Slade "reveal" that before she even met Slade, she murdered a good man in Africa.A few problems with this: 1) There's only have Slade's word for this, in-universe. That is, we only have the word of DeathstroketheTerminator. And in context, he's trying to gain Beast Boy's forgiveness so that Beast Boy stops trying to kill him, and the African man just so happens to be someone Beast Boy was close to as a child. 2) The same time Slade says this, he also says something demonstrably false, that Terra hated everyone, starting with her heroic brother, Geo-Force from the Outsiders. But in the TT/Outsiders Crossover where they reunited for the first time after joining their respective teams, she was openly affectionate to her brother, which she never was to the Titans. She even thought to herself that she didn't want her brother to go down with the Titans when she betrayed them, which means that her open affection wasn't an act. 3) Aside from this one uncorroborated (and never mentioned again) story from Slade, nothing else in her background suggests that she'd ever been to Africa.
It gets better: Slade said that he was offered money to kill the African guy, but refused because he was "a friend of his". If Terra killing him is true, then Slade really isn't making himself look good at all, seeing as he would have had hired and slept with the girl he knew damn well murdered his friend!
In Runaways, it is constantly said that Alex is a genius and master strategist to make up for the fact that he has no powers. An observant reader might have trouble believing this, considering all the tactical errors he makes (not grabbing money or food before running away, letting an untrusted stranger into their hide-out, etc). Subverted with the reveal that he's The Mole, and all those poor decisions make sense in context of manipulating the group to save his own parents.
The Sentry: As a consequence of being a walking Cosmic Retcon, he's a moral paragon (who has no compunction about executing people), more powerful than anyone in the universe (who doesn't seem that much stronger than Thor outside of instances of New Powers as the Plot Demands), friend of every hero and foe of every villain (who mostly ignore him whenever he's not on-panel) with a list of achievements a mile long (there was the time he killed Carnage and... uh...). This was all well and good when he was confined to a miniseries, but prolonged periods led to the more traditional response of Informed Ability.
Nightwing is supposed to be The Heart of the DC Universe, and holds a special place in the hearts of everyone. Older heroes see him as a surrogate son (The Ace kind who you show off at parties and the like), younger heroes see him as a Big Brother Mentor (he has lead most of them at one point or another) and civilians generally seem to consider him Batman's heir (which is actually true). In short: he is very friendly with everyone in the DC Universe, and as a result, when something happens to him, there is a uproar. He also has excellent leadership skills as a result of his experience in leading different teams. This actually worked in the Post-Crisis universe. In the New 52 Universe, Nightwing is not shown once interacting with heroes outside of the Bat-Family. In fact, in his brief appearance in Justice League, he comes off as incredibly anti-social. Hell, he has led one team which fell apart. Plus, with all the older heroes being, at max, in their 30s, and Dick being in his early 20s, the surrogate son idea is also removed. This leaves his Big Brother Mentor status, which only applies to subsequent Robins, which wouldn't even make him come off as friendly (in fact, it may make him come off like a snob if you think about it), let alone make him The Heart, as literally every Robin after him (in the New 52) is anti-social to a large degree. note Jason Todd is a wanted criminal and generally avoids other heroes and is not liked by them, Tim Drake is shown to have almost no friends and almost always kept to himself before founding the Titans, and even then, he's anti-social, and Damian Wayne has never interacted with people beyond the extended Bat-Family, who he's a Jerkass to
Doubles as a historical in-joke: the bronze-age greeks had two gods of war (or arguably more, but two primary ones) in their mythology: Athena and Ares, and they had a running in-mythology feud over who was better at it, with Athena sponsoring Athens and Ares sponsoring mostly Sparta while they flung continual insults at each other. Since the literature of Athens largely survived into the modern age and that of Sparta didn't attain the same significance, in a lot of the surviving mythology Athena is the calm, skilled strategist that wins by guile and true bravery in noble defense of her chosen people while Ares gets to play the dumb brute who just loves murdering things and doesn't do that particularly well.
Iron Man's "buster" armors are almost always hyped up as being designed and able to defeat a specific powerful threat - the Hulkbuster can defeat Hulk, the Phoenixbuster can defeat Phoenix, the Thorbuster can defeat Thor, etc. Unfortunately for Tony, pretty much every time a Buster armor goes up against a person it was made to fight, it fails, and often pretty easily - at best, it manages to get in a sucker punch and hold out for a brief period before he runs for it. The only time a "buster" defeated the opponent it was made to fight was in the film Avengers: Age of Ultron. It seems Tony has a very inflated opinion of his armor's abilities.