This is not to say that it is a bad movie, it's certainly not. It's good. It's just not wonderful, great, fantasic, earth-shattering and all the other adjectives being heaped on it. Having read the books and seen all the other movies I can say that RoTK (the movie) is very much like the Two Towers (the movie). They are enough alike as to be inter-changeable.
Having read the books over and over for some 25 years I was initially afraid to see the first of the movie trilogy. I thought it would be horrible and that it could never compete. I was VERY pleasantly surprised. These movies do justice to the books and are as good as movies of Tolkien's work have ever and perhaps will ever be. Taken as a whole, they are great blockbusters. They are far from horrible. Yet, they cannot compete with the books. The movies add some interesting things (Arwen as a main character - who knew Liv Tyler could act so well??) However, they remove several, to my mind, important things. No Tom Bombadil? (Read this article if you want a view as to why Tom was important)
Nothing, of course, can be perfect, and these movies are fine as they are, entertaining romps. Sean Astin, in particular, is perhaps the best part. They are not (and RoTK is certainly not) classics in the realm of Gone with the Wind, as I had read in the reviews.
Beyond that, RoTK is a distinct disappointment. I was expecting a thrilling and rousing conclusion. Instead, it sort of petered out. Perhaps that was because I knew how it ended, however, I took a friend of mine who had seen all the previous movies and had NOT read any of the books with me to RoTK and even he thought the ending was less then stellar. He agrees with me that The Two Towers and Return of The King could be one in the same movie, or at least integrated into each other with no one noticing. Over all, I was quite disappointed.