Some communities have a talent for stripping the context down to one stupidly perfect line. There's this bit from Terry Pratchett where Granny Weatherwax says, 'Personal isn't the same as important.' It gets used for everything—when someone's being overly dramatic in a ship war, when a main character is having a whiny monologue, when a reviewer gets too personal. It's the ultimate shutdown, weaponized by fans who've seen it all. It's funny because it's so dismissively wise.
Then there are the ones that are just pure, shared 'what the hell' moments. Like in 'Gideon the Ninth', when Gideon says, 'I'm going to the library to bone.' The quote itself is absurd, but the reaction chain it spawns is the real gold. You get the people who haven't read it yet posting the eyes emoji, the ones who have replying with 'oh honey, no', and the deep-cut memes about skeletons. It's less about the humor in the book and more about the communal inside joke it becomes, a badge for being in the club.