Accidental Innuendo: "Too true, muttered George Weasley. I haven't been properly dry since August."
Alternative Character Interpretation: Arthur Weasley, patriarch of the Weasley clan - loving husband and father who enjoys his Muggle studying hobbies a little too much, or a completely selfish and irresponsible man who made his large family suffer unnecessarily for years by way of a job that he clung too simply because it allowed him to indulge in his fascination of Muggle culture despite it not paying nearly enough to support his clan because he cared more about his hobby than he did about his wife and kids?
Hermione's Polyjuice Potion accident. Seriously, it's a bizarre incident that comes out of nowhere, it has nothing to do with the overall story, it has no real impact on the overall story, it adds nothing to the overall story or the characters, the fact that it takes so long to fix when considerably more fatal accidents are laughed off and fixed with relative ease just makes the health care of the wizarding world look ridiculous, etc.
Until Half-Blood Prince came out, this book was saddled with the reputation of being an altogether irrelevant episode.
Crosses the Line Twice: Peeves accusing Harry of killing students? Not funny. In song? Hilarious. Especially when he adds a dance routine.
According to The Other Wiki, several Christians — those who don't think that these books teach Satanism — compare the climax to John Bunyan's The Pilgrim's Progress, a pinnacle of Christian literature.
In his relationship with Ginny, it's easy to see Tom Riddle as a kind of metaphorical Internet predator. J. K. Rowling herself acknowledged in an interview on the DVD of the movie that the diary is really a lot like an Internet chat room, but said she hadn't been in one at the time she wrote it so it's just a coincidence. Rowling has also said her inspiration was the fact that she found diaries to be really scary, as a person's deepest darkest secrets are hidden in them. So rather than the focus of the danger being on talking to strangers, it's more on playing with something you don't understand.
Fridge Brilliance: Of course Lockhart would happily seize Ron's wand to try to cast a Memory Charm on him and Harry; of all of Ron's teachers, he's the only one that wouldn't have a clue that Ron's wand has been broken all school year because he's the only one who hasn't been teaching any actual magic.
When Harry first sees Ron's room at the Burrow, Scabbers is curled up asleep in a sunny spot. Real rats warily avoid direct sunlight, as it's too bright for their dark-adapted eyes and makes it easy for predators to spot them. But Scabbers has a human's preference for light.
"Funny Aneurysm" Moment: Although Gilderoy Lockhart's memory being wiped is sort of Played for Laughs, it becomes horrifying when we see him again in book 5 in the Spell Damage Ward at St. Mungo's. Also interesting, considering that the memory charms that Gilderoy Lockhart put on the people who actually did the deeds he claims he did are decidedly not played for laughs.
Genius Bonus: The bronze statue of a boar that has a prominent place in the Hogwarts entrance hall is a copy of Il Porcellino, a famous statue in Florence, said to have magical powers. The original Il Porcellino figures in one of Hans Christian Andersen's lesser known fairy tales, The Metal Hog, in which it comes to life and tells a poor street urchin that he is destined to become an artist - not unlike what happens to Harry Potter.
Harsher in Hindsight: The story behind Hagrid's expulsion from Hogwarts has become even more disturbing now that it's been revealed that something very similar happened to Newt Scamander. Even if one were to accept that no one believed in the legend of the Chamber of Secrets, how could you expect anyone who didn't know the real story to feel sympathy for Hagrid after the second monster attack in living memory - and this time it was actually deadly?
Remember when Harry less than fond of the Skele-Gro? Turns out his ancestor, Linfred of Stinchcombe, invented the stuff ages ago.
Snape looked "as if Christmas had been cancelled" when he finds out that Harry and Ron won't be expelled for flying the Ford Anglia to Hogwarts. He would later be portrayed by Alan Rickman in the films, who himself called off Christmas as the Sheriff of Nottingham in Robin Hood: Prince of Thieves.
The Reveal that Lockhart's "memoirs" are full of bullshit. Within ten years of the release of the book, similar reveals occurred regarding two memoirs (A Million Little Pieces by James Frey and Love and Consequences by Margaret Seltzer) that Crackedhas summarized as "lame-ass white kid wants to be a stone-cold criminal." In fact, Margaret Seltzer claimed to have used Lockhart's method of taking other people's life stories and passing them off as her own. A little after that, a Holocaust memoir was revealed to have been forged, but that arguably falls under Harsher in Hindsight or "Funny Aneurysm" Moment.
It's the Same, Now It Sucks: Sadly film critics other than Roger Ebert talk down to this film for taking a slow paced, Adaptation Distillation approach like the first film. After the third film came out and the tone of the film series changed profoundly, only purists mention this one. The book is also given very similar treatment.
It Was His Sled: There's a Basilisk in the Chamber of Secrets. Tom Riddle is young Lord Voldemort.
Lockhart is a fraud. This was fairly obvious at the start, but the reveal that he memory-wiped the real heroes was a surprise on the first reading (if only because before then, he came off as laughably incompetent and a huge Jerkass, but otherwise harmless).
Jerkass Woobie: Filch qualifies when his cat, Mrs. Norris, gets petrified. Also when it turns out he's unable to perform magic despite being wizard-born. No wonder the guy hates Hogwarts students so much.
Lucius Malfoy's plot in the second book amounts to this. Arthur Weasley is going to pass some law that makes it harder for him to be evil. So he puts in an old Dark Arts object cursed by Voldemort himself into Ginny Weasley's cauldron. He rids himself of evidence of his genuine loyalty to Voldemort and compromises Weasley's reputation when his own daughter gets possessed and attacks students. Oh and he uses that to get Dumbledore dismissed from Hogwarts. Even when Harry unearths the truth, Malfoy points out that he has no real evidence to pin the blame on him. Unfortunately for him, Voldemort returns and he finds out what happened to that diary he gave Lucius for safe keeping and as a sign of the latter's loyalty. It also contained Voldemort's Soul Jar but Malfoy didn't know that.
The chapter art for the polyjuice potion sequence, which resembles one of the middling stages of an Animorphs cover as Harry transforms
Squick: Harry getting his arm broken. Lockhart tries to fix him up, and the bones disappear. In the film version all that's left is a misshapen mess that makes an unnerving sound.
They Wasted a Perfectly Good Character: One Mugglenet book suggested the basilisk to be this. It's apparently intelligent, centuries old, and Harry can understand its speech, but it only shows up at the end and mostly acts like a generic monster, rather than giving the audience any signs of even being a character.
Ernie "They say the real reason You-Know-Who was after Potter was because he didn't want another Dark Lord competing with him."
What an Idiot: In the film, Lucius Malfoy attempts to cast the killing curse on Harry right outside of Dumbledore's office.
Blame Jason Isaacs for that one. When they filmed the scene, he said the first spell that came to his mind, as he'd only read the fourth book at the time. Oh, and it was an adlib.
Hermione mistaking a cat hair for a human hair leading to her unfortunate Polyjuice Potion accident - how the Hell can the smartest witch of her generation be dumb enough to mistake a cat hair for a human hair when considerably less intelligent people can tell the difference with ease?
Vernon Dursley. Do you not remember what happened the last time you tried to keep Harry from going to Hogwarts? He's lucky he only got a few teenaged Weasleys. Imagine if Hagrid - upset when giant man-eating spiders are harmed, mourn the loss of his baby dragon fiercely, three-headed dog raising, refuses to leave his literal-giant brother behind because the other giants are mean to him, Hagrid had found out that the Dursleys were locking Harry in his room and starving him. Exactly how many pieces of Vernon do you think would have been left?
Ron and Harry. When they can't get to the platform, instead of waiting for Mr. and Mrs. Weasley to return (or any wizard, really) and help them get to Hogwarts, they steal the car and try to get there on their own. Justified in that they're 12 and slightly panicked.
McGonagall actually points this out later, asking them why they didn't just write to her or Dumbledore and explain the problem. Harry privately realizes that that was an obvious solution that he didn't even think of.
Woolseyism: Voldemort's Significant Anagram name, revealed in this book, in the original was Tom Marvolo Riddle, an anagram of "I am Lord Voldemort." Translations changed various parts of his name; for example, in the German version, his name was Tom Vorlost Riddle, which becomes "... ist Lord Voldemort" (is Lord Voldemort). Something is gained in the German version particular here, as his middle name sounds an awful lot like Verlust, meaning "loss," which applies to Voldemort in a variety of ways.
The Spanish translation called it Tom Sorvolo Ryddle, to spell Soy Lord Voldemort. Of course, "Ryddle" isn't an English word, but a Spanish speaker could pronounce it the same way as the English name without noticing anything wrong.
Other languages aren't quite so lucky. Just ask Tom Elvis Jedusor, of the French translation.
Or Romeo G. Detlev Jr. in Denmark (The G. stands for Gåde = Riddle)
Russian translation got interesting with it: since the first two books were translated back-to-back, Tom's name had only two letters changed (and one removed since that's what Russian language does to silent e's) - becoming Tom Narvolo Reddl. It's Voldie's name that got changed - to Volan-de-Mort. And thus Tom Narvolo Reddl becomes Lord Volan-de-Mort. It creates a small continuity error in the sixth book, in which his and Marvolo Gaunt's name get their M back.