J.R.R. Tolkien's son Christopher nailed exactly what's wrong with the Lord of the Rings films in a recent interview:
"They eviscerated the book by making it an action movie for young people aged 15 to 25...The chasm between the beauty and seriousness of the work, and what it has become, has overwhelmed me. The commercialization has reduced the aesthetic and philosophical impact of the creation to nothing. There is only one solution for me: to turn my head away."You can read more here.
I remember going to the theater when Fellowship came out fully prepared for some changes in the story. After all, a film is not the same thing as a book. They are two different media. So naturally things will have to be cut, reordered, and altered to fit the film. However, the movies did far more than omit and reorder; the entire worldview and philosophy of the books were ditched for trite Hollywood clichés. Noble characters like Aragorn and Elrond became notably less noble. The relationship between Aragorn and Arwen went from a medieval courtship in the book to a "forbidden love" Harlequin Romance in the film. Gimli was exploited as mere comic relief. Frodo's strong moral character in the book was changed to a Byronic paleness accompanied by frequent fainting.
The problem can be summed up nicely by a quote from Tolkien's letters. Tolkien wrote, "The Lord of the Rings is of course a fundamentally religious and Catholic work..." Note the "of course" he threw in there. As if nothing could be more obvious. Unfortunately the films are neither religious nor Catholic. Nor do they share even remotely in such a worldview.
Let's look at some examples. Unless you really understand the Catholic veneration of Mary, you won't understand Gimli's veneration of Galadriel. It will come across, as it did in the movies, like some sort of bizarre crush. Nor will you understand the courtly veneration of women in general. Hence then change in Aragorn and Arwen's relationship. Without the Christian background, you've got an Aragorn who has "chosen the path of exile" rather than an Aragorn with a sense of driving focus and determination to fulfill his long-awaited destiny. You have a Christ who has given into temptation on the mountain. Evil seems much more pervasive and powerful in the film than in the book; the merriment and joy of places like the Prancing Pony have been replaced by scary atmosphere, and Tom Bombadil with his bright boots is nowhere to be seen. Finally, there is the matter of the Ring itself. In the worldview of the book, original sin is innate in humans. The Ring simply reveals and amplifies the sin of Pride and desire for power. Thus characters who have no desire for power are not tempted by the Ring, i.e.- Faramir. The film, on the other hand, participates in the modern worldview that people are all good at heart and so the Ring makes them evil. Hence, no one is immune from the Ring, and Faramir becomes a class A jerkface in the film because of it.
I would still like to see a Lord of the Rings movie that fully participates in a pre-modern, medieval, Christian worldview the way the books did. The Peter Jackson movies got all the externals right, oftentimes stunningly so. But the visuals are just a veneer and the heart of the matter was left out. They look medieval on the outside, but they are modern to the core.