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.
CGI Geoffrey Rush: —your ring allows you to conjure up any object you desire. The only limit to a Green Lantern’s power is his imagination. Ryan Reynolds: Okay, I imagine a world without evil. CGI Geoffrey Rush: The only limit to a Green Lantern’s power is the screenwriter’s imagination. Ryan Reynolds: Oh. Look, I made swords and guns!
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.
Batman 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 that made it look like Vesper was killed because she discovered Bruce is Batman which really threw some doubt on the situation.
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.
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.
Megatron from All Hail Megatron claims to be an amazing tactical genius, but his abilities do not even come close to living up to the hype.
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 from Spider-Man. 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. Writers think we're going to like her because she's an odd mash-up of former love interests' good qualities, except all those qualities are told not shown due to said short publishing time she's had (All those others? At least a decade of publishing each, movie appearances for two of them, and at least one animated series).
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.
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.
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.
It's worse than that, actually. 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, 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.
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 basically is this trope. 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 to Informed Ability.
Minor Batman rogue 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 and shouting "Catch!" wasn't going to live up to that hype.