Ass Pull: Whilst in the Maelstrom, the protagonist writes about how no ship has ever escaped the vortex. He is knocked unconscious, and when he comes to, he and his friends are safe, and offers no explanation as to how that was achieved.
— "What happened that night, how the skiff escaped from the Maelstrom’s fearsome eddies, how Ned Land, Conseil, and I got out of that whirlpool, I’m unable to say. But when I regained consciousness, I was lying in a fisherman’s hut on one of the Lofoten Islands. My two companions, safe and sound, were at my bedside clasping my hands."
The Nautilus crew didn't have an epic showdown with a giant squid, they had a prolonged battle with an entire school of giant (but not as large as the one seen in the film) squid. The Disney film contributes a lot to this misconception, since (presumably) the studio only had enough money in the budget for one giant animatronic squid. That, and/or it was Walt Disney's way to cash in on the 1950's giant monster movie craze.
Related to the above: it's somewhat debatable whether Verne actually meant his famous monsters to be squid; in the original French text, he referred to them as "poulpes" ("octopuses") rather than "calmars" ("squids"), and many early English translations likewise called them "poulps" (an archaic English term for octopuses). This may have been in the interest of greater scientific accuracy: in the years since the book was published, zoologists have become mostly certain that Real Life octopuses don't actually grow to the gigantic proportions seen in the book, though Real Life squid do.
The most common image of the Nautilus has her equipped with a huge saw-like raking blade, and this is often said to be Verne's original idea. In fact, this was designed by Harper Goff for the Disney movie. Verne's Nautilus was said to be cigar-shaped and have a ramming prow (in fact, its description is almost identical to modern military submarines, complete with conning towers and dive planes).
Also due to Disneyfication and James Mason's memorable portrayal, many people's image of Captain Nemo is as a dark-haired/bearded Caucasian man of presumable East European heritage rather than the Indian prince he is in Verne's novel. Understandably, this leaves viewers/readers of The League of Extraordinary Gentlemen scratching their heads, since both the film and the comic series it's based on stay true(r) to Nemo's literary depiction.
The title refers to the distance that the Nautilus travels while under the sea, not the depth that it dives to (20,000 leagues is actually about twice the circumference of the Earth). There was actually a Saturday Night Live sketch where Kelsey Grammer as Nemo vainly tries to get the others to comprehend that they're not that deep. The title is more clear in the original French, where it says "Under the Seas"
Captain Nemo is the antagonist of the novel, not the protagonist. Though he's certainly the most famous character in the novel, he's an Antihero at best, and a full-on villain at worst. This misperception is probably because Aronnax is the Unreliable Narrator who idolizes Nemo before he fully understands what's happening.
He has something of a Misaimed Fandom in good part because of his wonderful toys— you read ocean explorers like Robert Ballard and Jacques Cousteau saying that as children, they read the book over and over and "wanted to be Captain Nemo", meaning they wanted his awesome submarine and diving gear, not that they wanted to be supervillains, and two later real-life submarines were named "Nautilus" in homage.
Fair for Its Day: The depiction of the Papua New Guinea natives is no better than you'd expect it to be for a book written by a European in the 1800s, but it nevertheless at least acknowledges it would be senseless to get the natives killed or injured just for not being able to understand the technology of the submarine. The book also just in general contains a very anti-imperialist slant. Along with Nemo's sympathy and assistance towards people in oppressed nations, even Arronax refers to the likes of Cortez as conquerors and oppressors.
First Installment Wins: The sequel to the book, The Mysterious Island is actually considered an excellent book in its own right but it's far more obscure than the first novel.
Funny Moments: Conseil hopes Ned Land will get some red meat soon, "lest sir wake up one morning and find only chunks of me to serve him".
Genius Bonus: Nemo's chosen pseudonym, which is Latin for "Nobody", is also the Latin form of the Greek word "Οὖτις"—which is the pseudonym that Odysseus used in the The Odyssey when facing Polyphemus the cyclops. It's an appropriate name for a man forced to endlessly wander the seas after being driven from his homeland.
A lot of the descriptions of whaling and fishing and the beauty of the sea becomes this due to modern pollution problems, rampant overfishing, and climate change. Worst of all, some species described in the book (the greater Auk, for example) are outright extinct. (Granted, the Auk was extinct then, too.)
Verne accurately predicted the high speed and secret conduct of today's attack subs, and the need to surface frequently for fresh air (with diesel subs). He also saw the danger they presented to the Royal Navy.
The Nautilus can stay underwater for long periods, allowing for extended secret operations - just like today's nuclear submarines.
It Was His Sled: There is no sea monster, just a giant, secret, highly-advanced submarine. This is meant to be a twist reveal, but is one of the most famous things about the story nowadays.
Magnificent Bastard: Captain Nemo, born Prince Dakkar, is revealed as the son of an Indian Raj whose family was slaughtered for his rebellion against the British. Nemo has since abandoned civilization to create his own by his brilliance and force of will, resulting in the Nautilus-an ingenious submersible that Nemo travels the world with, striking fear into the hearts of the British by destroying their ships. Nemo is polite even to his prisoners, taking everything they need from the sea and their own victims, while maintaining a air of civility and geniality, showcasing his unmistakable charisma and drive.
The crew spend an entire chapter killing sperm whales for no particular reason, except that Nemo doesn't like them. That being said, the action is treated as ambiguous in-universe, with Ned Land, a whale hunter himself, decrying it as butchery.
Several times they find endangered species, such as a sea otter and a dugong, and kill them. Even when they point out how bad things are going to be if humans don't stop killing and eating endangered animals, they still proceed to do that exact thing with little remorse. And the hunting of a supposed undiscovered species of "giant narwhal" that starts the plot of the book in the first place, as well.
The crew also treat Papuan natives who attack the Nautilus like savages instead of people; they don't even stop one from wandering onto the (electrified) staircase of the ship.
However, only Ned Land, Conseil, and Aronnax treat them as savages. Maybe could be said that it was Fair for Its Day? Captain Nemo lampshades that "savages" can be found at any part of the world, and even when the Papuans wandered on the electrified staircase, it's stated it was only capable of repelling them and not killing them. Nemo said:
"Well, sir, let them come. I see no reason for hindering them. After all, these Papuans are poor creatures, and I am unwilling that my visit to the island should cost the life of a single one of these wretches."
Writer-Induced Fanon: Nemo's ethnicity as an Indian, a Sikh Prince to be exact, was made explicit in the novel's sequel, The Mysterious Island but was left ambiguous by Verne in the first novel. There are many in-text hints such as when Captain Nemo rescues an Indian sailor and admits that he sees him as belonging to the country of the oppressed and considers him a countryman but it's still quite cryptic. Most adaptations made Nemo a European. Verne did intend him to be Polish originally although he was advised against it by his publisher. Still, it was Verne's own idea to eventually make him a non-European.
The 1954 film adaptation contains examples of:
Designated Hero: Ned Land is kind of a jerkass throughout the film, at least to their "host". Though when we see how bad Nemo really is, he seems a little justified.
Designated Villain: Nemo comes cross as this at first — until you see his true colors. Ned seems more than a little justified in mistrusting him after he sinks a ship full of innocent sailors. This can, in large part, be blamed on James Mason's charismatic performance.
The dishes which thoroughly revolted Ned Land and Conseil (and also 1954 American audiences) at dinner, are routinely found today in most Japanese and other East Asian restaurants everywhere; i.e. sea snake is eel, unborn octopus is either baby octopus or fish eggs, and sea cucumber is still called sea cucumber (but still no whale milk, anywhere so far).
"Whale of a Tale" becomes funnier when you know that Disney did introduce a Mermaid Minnie at one of their parks.
Nightmare Fuel: The sight of the well lit Nautilus speeding towards their ships. Made even scarier when you see the fanatical and hateful glare of Captain Nemo as he rams a ship in one scene.
Older Than They Think: People often forget that there were other film adaptations of the book made around fifty years before this one, when the motion picture business was still in its infancy. The earliest one was a silent short film made in 1905, but there were other adaptations made in 1907, 1916, and 1917.
Ooh, Me Accent's Slipping: Paul Lukas begins the movie with an affected French accent, but towards the end, the accent sounds more like his native Hungarian.
Values Dissonance: Nemo's heritage was a little vague but he was eventually revealed to be of South Asian ancestry. Nemo being portrayed by a white actor would nowadays be derided as whitewashing. note Disney's portrayal of Captain Nemo seems closer to how Jules Verne imagined him originally. He initially wrote Nemo's character to be a dispossessed Polish aristocrat, but because of the diplomatic situation between France and Imperial Russia at the time of writing, Verne's editor asked him to reconsider.
Visual Effects of Awesome: The battle with the Giant Squid. Even today it looks awesome. Which is kind of funny, as the battle as it was initially shot (in daylight with calm seas) was very much the inverse of this trope. A little darkness and splashing water to cover the obviousness of the huge puppet, and you have a classic. Compare this to the finished product.
, and then an underwater "utopia" was introduced...