Some people pointed to moments when a specific scene was omitted from a movie. Often, the hope of that scene on the big screen brought them into the theatre, only to leave disappointed.

Dinosaur Secrets 

"The most cinematic moment of Jurassic park was when dr Satler was trapped on the roof with monsters coming and used math to calculate that she could leap into the pool. Not in the movie." -- Dr-P-Ossoff

"Jurassic Park. In the book, you find out exactly what's making the Triceratops sick, and it's a whole thing." -- wscuraiii

No Love for the Old Lady 

"As much as I love the film version of 'To Kill A Mockingbird' (and trust me, it is one of my favourite films of all time), I really wish they had kept in the section about Jem getting to know the angry old lady from down the road that complained at the kids every day."

"Jem spending the summer reading to her was a nice part of the book, and I wish it was shown on screen."

-- YellowRainLine

Harry, Stealing the Show 

"The part in the first harry potter book where they have to go through puzzles to get to the sorcerers stone."

"They completely take out one of the tasks that Hermione helps Harry figure out!"

-- Lainnnn

Letting a Great Bit Go 

"The scene in The Martian where the dude on earth is wondering what the astronaut stuck on mars must be thinking out there all alone not knowing if anyone else knows if he's alive and it cuts to the astronauts log where he's like 'how can Aquaman control whales? They're mammals' "

-- liontoaslaughter

Other people discussed how characters were treated in film adaptations. They couldn't believe what little--or misplaced--development and backstory was given to their favorite people in the story.

Three Cheers for Kaa

"Every time they do The Jungle Book, they do Kaa so fu**ing dirty. Every time."

"In the books, Kaa is on the same level as Bagheera and Baloo, one of Mowgli's guardians and teachers. But Walt Disney (and Western culture in general) had the whole 'snakes=bad!' mindset and so they make him a laughable villain."

"And he's a fu**ing bada** in the books!"

"Mowgli gets kidnapped by the bandar-log (monkeys) who are a bunch of curious indecisive morons, but have overwhelming numbers. Baloo and Bagheera try to save him but even they get overwhelmed."

"Then Kaa shows up, apparently the only thing the monkeys fear, and he straight up hypnotizes them all. Mowgli and his buds escape and Kaa's all like 'I'll catch up with you later' and it's heavily implied he's about to eat a sh**-ton of monkeys."

-- herculesmeowlligan

A Nuanced Struggle 

"I've seen a lot of people mention the common scenes from LOTR but one that always seems to be left out of these conversations is the fact that, in the book, Denethor had a palantir and had been using it to essentially play mental tug of war with Sauron for years trying to get intelligence, which eventually led to his madness."

"For example, Sauron would show Denethor truths but very deceptively, such as showing him the black sails of the Corsairs of Umbar sailing to Minas Tirith, but not showing that Aragorn had captured them."

"It made his character a lot more sympathetic and tragic, and it made sense since the palantiri had been established already."

-- J71919

Across the Board 

"Enders game. All the character development was missing." -- seventeencans

"The pacing was so fast too that absolutely nothing can sink in before the next big thing is happening. The casting acting and aesthetic were all totally fine. That same everything would have worked if it were just a 12 hour hbo series instead. They could even technically tell enders shadow concurrently if they did it right..." -- SARAH__LYNN

Finally, there were some film adaptations that seemed to miss the point altogether. Or perhaps they chose to hammer home a different moral.

Either way, fans found themselves a bit deflated after seeing these ones.

More Calculated 

"The movie version of One Flew Over The Cuckoo's Nest had to cut out a lot but I think the one that hurt the film the most was less of a scene and more of an explanation. (Spoilers) After Billy dies and everyone realizes the control nurse Ratched had over them, McMurphy realizes there's only one thing he can do that will make them see her for what she is."

"It's very clear in the book that he was consciously choosing death by lobotomy to save his friends from eternal abuse. In the film it just seems like he's angry."

-- namelynamerson

Watered Down 

"Neverending story. Admittedly it's been a while since I've read it, but the movie is the first half of the book. The point of the movie is, wimpy kid gets his wishes come to life through imagination and everything is possible and he brings back Fantasia to life."

"In the book, after he goes to Fantasia, he starts forgetting the real world and forgets who he was, and the point was that too much make believe is not good."

-- nullrecord

Roasted 

"Artemis Fowl. They left out the pivotal scene where we get a good movie."

"In the books, said scene is between page one and the last page."

-- DvDCover