Mandy Patinkin called it "The Wizard of Oz of our generation." Conceived as a fairy tale by William Goldman, TPB is a love story, a comedy, a fable, and a swashbuckler by turns. Modestly successful during its theatrical run, the movie became an icon following its release on video, allowing it to reach a wider audience that now stretches across generations.
It's a brilliant, timeless movie which is preceived as representative of the cape-and-sword genre rather than the loving satire of that genre it was intended to be.
TPB reached a broad audience at a time when cape-and-sword movies were few and far between. The last significantly successful swashbucklers in the US, Richard Lester's Musketeers movies, were thirteen years earlier, in 1974, and Stephen Herek's The Three Musketeers wouldn't premiere for another five years, so for many viewers, particularly younger viewers, TPB was their introduction to the genre. With its relatively mild violence and the chaste relationship between Westley and Buttercup, TPB likely reached its audience at a younger age than the Musketeers movies as well.
And as a result, the caricature became the reality.
Cape-and-sword books are often darker and grittier than movie swashbucklers. The Three Musketeers, in its English-language versions, was often sanitized for its audience. For example, to avoid impuning the Catholic Church, the comte de Rochefort, not the Cardinal, becomes the instigator of the plot against the Queen in Rowland Lee's 1935 movie, whereas Richelieu is a duke, not a prelate, in George Sidney's 1948 film - that's the one starirng Gene Kelly. D'Artagnan's escapades with Constance Bonacieux and Milady were whitewashed as well - even the Richard Lester movie, the most faithful to the book of any of the films in English, omits d'Artagnan disguising himself as the comte de Wardes to seduce Milady, though it does include the Gascon's seductions of the married Mme Bonacieux and Milady's maid, Kitty, as well as Milady herself.
Swashbuckling movies generally focus on fast-moving action and frequently slapstick humor and this was no less true of the many cape-and-sword films through the first half of the twentieth century. The action in TPB is an homage to the era of Errol Flynn. What was different is that the era of swashbukling movies wasn't so far removed from the time when cape-and-sword stories were more common on bookshelves as well. Swashbucklers fell out of style at the same time cinema became more frank in its depictions of both violence and sexuality, so the darker side of cape-and-sword stories familiar to an audience which read the books was lost on those whose primary exposure was through the movies.
By the time we reach TPB in 1987, neither cape-and-sword books nor movies are as ingrained in popular culture, and the exploits of Westley, Inigo Montoya, Fezzik and the rest shape the perceptions of what a swashbuckling tale should be for a generation with little exposure to the depth and breadth of the tales which the movie so affectionately satirizes.
Personally I find it impossible to imagine Prince Humperdink disguising himself as Westley to seduce Buttercup, or a drunken Inigo picking a fight with a villager just to watch him die, or Vizzini strangling Buttercup with a string of rosary beads, or Fezzik crushed under a castle portcullis.
But those are part of cape-and-sword tales, too, alongside the witty one-liners and flashy stunts. In swashbuckling stories, the heroes may die, insults may go unavenged, and the girl can end up dead on the floor of a convent.