Like many people, I get an initial feeling of dread when I hear that a book I like has been turned into a film. Sometimes I won't even watch the film even though I love the story! This is a sentiment shared by many and for good reasons too. Here are, what I believe to be, the main problems with films adapted from books.
Firstly, the characters are NEVER as you imagined them. Even if a novel provides a clear physical description and detailed analysis of all of their quirks and characteristics, the character is always different. It can also be difficult when a well-known celebrity is playing the role as it is hard to separate the real-person and the character.
Secondly, if a novel is long or filled with lots of action and events it is generally impossible to condense it down into a two hour long film without cutting parts out. Of course, nowadays it has become common to split a novel into two parts, making two films. However, if there is an event in the novel that does not necessarily progress the narrative and is not something that will appeal to the majority of the audience, then it is likely to be cut from the story completely.
Thirdly, filmmakers like to use a bit of artistic license, and can sometimes change a storyline completely. This can be to suit the audience the film is being aimed at or just because they feel their interpretation is more appropriate.
Finally, a film is just one interpretation of a novel. If a novel has been read by a million people then that means there are a million versions of that novel floating around in people's minds. Therefore it is extremely improbable that the filmmaker has the same interpretation as you, or the other 999,999 readers.
It can be annoying watching a film that completely changes a story you like. The recent, award-winning film, Silver Linings Playbook, for example, was based on a book I love. It was changed an awful lot from the book, with various characters and events completely altered. However, despite being incredibly different, it maintained the same positive, feel-good mood of the book, making it really enjoyable. Do not be deterred from watching an adaptation, just because it might be different does not mean that you will definitely not enjoy it.
Have you read any books that have been made into films? Which version did you prefer, the book or the film?