Everyone goes into a film adaptation of a novel expecting some divergence. For the most part, director Peter Jackson was true to Tolkien's beloved tale and expansive universe when he made the Lord of the Rings trilogy. Even so, there were things left out of the Lord of the Rings films, and a few of the lost details are rather massive.
While some devout readers hate the LOTR movies for these changes, a number of the creative choices Jackson and co-writers Fran Walsh and Philippa Boyens made are understandable. But some revisions make the story more convoluted, leaving fans to wonder what went into those decisions. Most people were probably okay without the ever-singing Tom Bombadil, but the final battle at the Shire was more sorely missed. With more than nine hours of screen time, it's amazing how many important details Jackson's LOTR trilogy is missing - and knowing the full story leads to a better appreciation of Tolkien's genius.
Aragorn's Fellow Dúnedain Descendants Are Missing
As a Ranger of the North, Aragorn's job entails more than just wandering around. He is a descendant of the Dúnedain, an ancient race of Men whose purpose is to stalk the free lands, smiting evil wherever they can. They are superior warriors and live three times as long as normal humans, which explains why Aragorn is 87 years old in the movies despite looking relatively young.
In the books, Aragorn is the leader of the Rangers of the North, and the Grey Company (a division within the group) assists him throughout the War of the Ring. From The Two Towers on, the Grey Company supports basically everything Aragorn, Gimli, and Legolas do, but the group is totally absent from the films.
The Shire Isn't Taken Over By Saruman At The End
In the movies, the Hobbits live happily ever after (for the most part) once the War of the Ring ends, but that's not what happens in the books. Originally, Frodo and his fellow Hobbits return to the Shire to find it's been overrun by Saruman (operating under the name Sharkey) and his cronies. They're thrust into the Battle of Bywater, wherein the four returning heroes lead their fellow Hobbits into battle against Saruman.
Considering how long the extended cut of The Return of the King already is, it's no surprise this part is missing from the film. But the vision Frodo sees in the Mirror of Galadriel does allude to it.
Arwen's Brothers Are Nowhere To Be Seen
The movies feature Elrond's daughter, Arwen, but there's no mention of the Lord of Rivendell's twin sons, Elrohir and Elladan. In the books, not only are they companions of Aragorn's father, they also work closely with the Rangers of the North (and Aragorn himself) prior to the War of the Ring. During the war, they accompany Aragorn through pretty much all the events of The Return of the King. These twin brothers stick by Aragorn's side all the way to the bitter end, yet Legolas is the only Elf depicted in the film's final battle.
Tom Bombadil Is Nowhere To Be Seen
In a world filled with strange, enigmatic characters, Tom Bombadil stands above the rest. The Elves call him Iarwain Ben-adar, which means "Oldest and Fatherless" in the Common tongue, and he may indeed be the oldest being in the world of Arda. He lives right by the Shire, and in the books, Frodo and his Hobbit companions run into him shortly after entering the Old Forest.
He saves Merry and Pippin from Old Man Willow (who's also missing in the films) and hosts the Hobbits in his home. What's more, Bombadil is completely impervious to the One Ring's effects, and it doesn't make him invisible when he puts it on.