The debates and issues surrounding book to movie adaptation is unique to the modern era, especially now that modern technology allows artists to create virtually anything that's appeared in ancient or modern literature. The sword and sandal movies of earlier eras were the first to take on the daunting task of adapting fantasy to film, even if the text was from classical sources, and today some of the biggest and most profitable franchises have a fantasy plot and setting.
The grandfather of all modern fantasy is Tolkien's The Lord of the Rings, and nobody was surprised when live-action movies were planned at the turn of the century. The surprise was how great they were, and fans of both books and films were impressed with the work Peter Jackson did with what must have been one of the most challenging literary adaptations to date. Not everything was better than Tolkien, of course, but it has to be said that Jackson came pretty close.
Books Are Better - Exposition
This might be too obvious, and the movie does a great job compensating, but those who have only seen the movie only have a small idea of how complex this story really is. When reading the books, it's obvious that this epic tale goes back eons, all the way to the romance of Beren and Luthien when Sauron was one of the benevolent gods. The movies actually start near the end of a legend that goes back to the beginning of time. It's the kind of gravitas that only several volumes of paper can give to a story.
It’s The Films - The Power of Gandalf
In the books, Gandalf is constantly kicking ass and taking names with his amazing powers, plus he's not human but a supernatural being, so nothing he does is a surprise. The movies don't tell us exactly who Gandalf is, but they humanize him a lot more than the books did, which made him more of an aloof superhero. As one of the main characters of the movie, that's a change the writers were compelled to make, and they did such a great job that Tolkien himself would be proud.
Books Are Better - Sam the Hero
Fans of the movies debate whether or not Sam is the true main character and hero of the whole story. He has more competition with his companions on screen. That debate was over in the literary world a long time ago, because everyone knows it's Samwise.
Most of the story in the books is even told from Sam's perspective. There's the saga of Bill the Pony, who was thought to be lost outside of Moria, the journey through the Dead Marshes, and the brief time when he carried the ring himself. It's worthy to note that Sam, being a Ringbearer as well, was offered a place on the same boat that carried Frodo to the Grey Havens. He refused because there's no place like home, which is always the last destination for the classic hero.
It's the Films - Comedy
Not to say that the book doesn't elicit a few laughs, most of them coming from Pippin, but movies need moments of levity more than books do. Pippin retains his status as comic relief, and he plays off his more grounded friend Merry, and the team works really well. Other moments of memorable comedy, like Elrond's priceless "How many Hobbits are hiding in my council meeting room" face don't appear in the books, but it's difficult to imagine the movie without them.
Books Are Better - The Stewards of Gondor
The movies reduce the last Stewards of Gondor to either cartoon villains or simpering kids when in the books they were actually a much more complex family. Boromir is the one exception here, and that's more about Sean Bean's screen presence than good writing. The character of Faramir actually becomes Tolkien's voice in the book, questioning the morality and necessity of warfare in general, and he is the only human who rejects the ring as easily as the Hobbits. Denethor and Pippin actually form a touching bond in the book, which is lost behind the Steward's obsessive, tyrannical attitude in the movies.
It's the Films - The Temptation of the Ring
Similar to the scenes in the book that focus on the Nazgul or even Sauron himself, the visual depictions of certain characters tempted by the ring are consistently impressive on the big screen. The exception would be Faramir's temptation, which is silly and cheap because it's not in the book at all.
Boromir, Gandalf, and Galadriel are all examples of figures of great power who were tempted by the One Ring. Boromir's journey is the most compelling, as he battles with himself over a long period of time as he travels with Frodo, whereas Galadriel has a more immediate and dramatic way of refusing the call. Gandalf's temptation could be the most terrifying, including a vision of Sauron and a harrowing jump scare.
Books Are Better - The Ents
The writers of the movies seemed to think that tacking on the "reluctant hero" trope on the Ents was a good idea. Instead, it's just a tiresome way of trying to surprise an audience who already knows where this is going. It was impossible for Treebeard not to know what Saruman had already done to the forest anyway, so this is just bad writing. Tolkien knew better, which is why he made the decision of the Ents to fight against Sauron a foregone conclusion and concentrated on the important stuff, like how that was going to happen, and where did the Entwives go?
It's the Films - The Nine
It turns out fantasy and horror make a great pairing, something that movie makers have learned from the success of the zombie movie genre. Although the Ringwraiths were chilling in the books, something about seeing them on a big screen and hearing their voices in stereo makes them even more terrifying. The musical score that accompanies their on-screen appearances is also enough to make your hair stand on end. In the books, they are brushed aside a little too easily initially and don't appear again until the forces of Mordor move on Minas Tirith.
Books Are Better - The Battle at Helm's Deep
By the time the second movie rolls around, the set pieces start to get bigger, which is why there are so many armies crammed into the Battle at Helm's Deep in The Two Towers. It's a familiar look, and it happens when producers and executives get all excited about putting as many characters in that one big scene as possible.
It certainly looks familiar to the Marvel cinematic universe, but that's not what happened in The Lord of the Rings books. Even when the film was released, this was too busy, putting the emphasis on the epic battle while totally forgetting the human drama taking place behind the walls, where the real action always was.
It's the Films - Arwen Evenstar
In the books, Arwen doesn't even get a chance to be boring. Occasionally she gets a mention to the reader while she quietly does her embroidery, nestled in her father's castle, just like a proper maiden waiting for her betrothed to prove himself. The movie makes their relationship precarious but in the books it's a done deal from the start, putting Aragorn's flirtation with Ewoyn in an even worse light. It's refreshing to see this flipped in the movies, where she appears with some adventuring experience herself and more to do than wait for Aragorn to reclaim his throne.