Of course there are some adaptations that are done well, but there will always be the complainers, the ones who were mad that a minor scene or line was left out or how a character did not look like how they are described in the book.
The author could have stated plainly in the book that so-and-so looks exactly like Taylor Lautner, but you will always have some readers who picture them differently. This comes from varying interpretations of the descriptions or how the character acts. They could be depicted physically in one way but their personality causes the reader to think of them in another way.
Another thing that will forever bother readers is the time constraint. Taking a huge novel and condensing it into a two to two and half hour movie is a challenge that most viewers seem to forget. Certain scenes are regrettably cut, but when the producers have to choose between telling the story and making sure everything ever mentioned in the book is filmed. When it comes down to it, the answer is obvious.
Lately, movies have tried to correct this by making movies into parts such as "Breaking Dawn - Part 1" and "Part 2" and "The Hobbit - An Unexpected Journey" so they can cover more of the events in the book, while making more money on their part too since they can now take a single work and turn it into a trilogy if they want to.
One problem that can also be encountered is production. Depending on the technology available, some adaptations suffer for poor visual effects that try to convey the fantastical setting of the book. While it is not a book to movie adaptation, James Cameron's "Avatar" movie is a good example of waiting for the technology to catch up with the concept. He originally came up with the story in 1994, but it wasn't until 2009 that the movie was released.
The number one reason why there will never be a perfect adaption is the fact that everyone pictures it differently. While some movies, like the "Harry Potter" series, do a good overall job at capturing the feel and magic of Hogwarts, every reader constructs their own version of the world they all love.
An example is a map of Panem, from "The Hunger Games" series. If you were to look for a map of Panem you would encounter several different interpretations. Some are similar, but none are exactly alike.
|These are just a few of the examples of readers different opinions of what Panem looks like|
The point is that people will never be satisfied with the movie version, if they have read the book, because in the translation of words to visuals the intangible pleasure of creating your own images is lost.