Adaptation Displacement: Many who can quote the movie from end-to-end don't even realize there is a book. In the introduction to more recent printings of the book, William Goldman says flat-out that if you are picking it up for the first time, it's probably because you've seen the film.
Asspull: The holocaust cloak. It strains the limits of credulity as to why Miracle Max (played by 5'7", 130 lb. Billy Crystal) would have possession of a cloak big enough to fit Fezzik (played by 7'1", 500 lb. André the Giant). Part of the reason it works is that the movie plays the absurdity for comedy. (The book mentions it briefly beforehand, Goldman claiming that Fezzik had used it in the - nonexistent - longer version of the chapter of "S. Morgenstern's" work.)
Also the soundtrack, courtesy of Mark Knopfler. In the last sword fight, the timing of the music is perfect with the choreography, and the chords embody the dramatic tension and climax of the scene. This happens throughout the movie, but here it is most noticeable.
Captain Obvious Reveal: The Dread Pirate Roberts as Westley. Buttercup even tells him that Westley had "eyes like the sea after a storm." The camera zooms on "Roberts" sea-blue eyes as she's saying it.
Cult Classic: Only made $30.8 million on a $16 million budget, but has become so popular that it borders on being an outright (non-cult) classic.
Girl Show Ghetto: This was part of the reason why the movie didn't do well on its first release. The marketing department at 20th Century Fox had no idea how to promote an absurdist swashbuckling adventure with a "girly" title and romantic arc and simply gave up and didn't release any promotional materials. Later, to compensate for the feminine-sounding title, some DVDs have a cover that puts the Dread Pirate Roberts in the front and center and emphasizes action in the synopsis, basically the exact same way the Grandfather talks it up to his grandson. On the other hand, some DVDs have Buttercup with a crown on the cover, with a very pink box, playing this trope perfectly straight.
In the 1973 book, gamers will appreciate Goldman deciding to skip over the long and repetitiveFetch Quest Inigo and Fezzik undertake late in the book to procure the ingredients for Max's miracle pill. In fact, this is just one of many things that was abridged that sounds an awful lot like Fake Longevity in an RPG game specifically, as opposed to just general story-telling Padding.
The Dread Pirate Roberts/Westley fights Fezzik, and claims that masks are "terribly comfortable. I suspect everyone will be wearing them in the future." Fast forward to 2017, and hecouldwellberight.
Hollywood Homely: Westley (among others) refer to Prince Humperdinck, played by the rather handsome Chris Sarandon, as ugly, specifically as a "warthog-faced buffoon".
Ho Yay: Humperdinck and Count Rugen anyone? And Inigo and Fezzik.
Informed Wrongness: Buttercup getting engaged to Humperdink, even though a) it was five years after she believed Westley was dead, b) Westley didn't bother to let her know he was alive or give her a reason to wait for him, c) her fiance was the crowned prince of Florin and could have her imprisoned or executed for saying no (which he does threaten to do in the book).
The Man in Black is Westley, and he has developed an immunity to iocane powder. Even if you don't know going in, Cary Elwes' now quite noticeable voice will probably tip you off. To his credit, Elwes at least tries to help the deception along by speaking softly and slowly early on, and in a much louder, faster, and more confident voice once he shows up as "Roberts".
Prince Humperdinck is the true Big Bad of the story. He doesn't love Buttercup and wants to kill her in order to justify a war against the neighbouring country. This fact is so much common knowledge that it's easy to forget that, in the movie proper, it's a twist that's only revealed after the death of Vizzini.
Many viewers take the "True Love" aspect of Westley's and Buttercup's relationship seriously, not seeming to realize it's a PARODY of classic "World's Most Beautiful Woman + Swashbuckler hero" romances, given how appearance-based their love is note Westley only ever lists Buttercup's "beauty and faithfulness" as his reason for liking her, and Buttercup calls him "poor and perfect, with eyes like the sea after a storm", and how little they have in common or get along when they aren't pining over each other from a distance.
A number of real-live couples model their wedding after Buttercup and Humperdink's royal wedding with the priest with the speech impediment, even though, you know... she's been coerced to marry the Big Bad, who intends to strangle her right after the ceremony.
Parodied in the book, with an explanation that a scene with Fezzik and Inigo going on minor quests to save Westley was 'cut' because it seemed like a ripoff of The Wonderful Wizard of Oz, even though the 'original version' came out first.
Vizzini saying "All aboard!". The book notes that this was before trains, but the saying actually comes from carpenters loading lumber, and this was well after carpenters.
Seinfeld Is Unfunny: The movie doesn't carry the same bite for people who grew up watching genre movies made afterwards, that don't feature or lampshade elements this one parodied and/or played straight.
When the heroes leave on the four white horses, it is quite clear that Fezzik has a body double. Necessary, though, in that there isn't likely a horse in the world that could carry André the Giant's mass.
The dummy in the holocaust cloak in the far shots of the "Dread Pirate Roberts" is particularly bad.
Fezzik. Not so much in the movie, but after learning that in the book his parents forced him into wrestling at the age of nine and threatened to leave him forever if he wouldn't fight, you start to feel sorry for him.
Similarly, Inigo. He grew up dirt-poor, and only had his father. Then, a nobleman kills his father for taking offense when the nobleman refuses to pay more than a tenth of the agreed-upon price for the sword he commissioned. Not only that, but in the book, plenty of villagers knew the nobleman murdered Domingo Montoya. They all just let him leave though, because there was no way they could convict him of his crime.