Harry PotterLet's start with the highest grossing book/movie series ever. Of course I'm talking about Harry Potter. JK Rowling's creation captured the imaginations of children and adults alike, breaking all of the world records you could throw at it. From 'The Sorcerer's Stone' to 'The Deathly Hallows', the Harry Potter series experienced unprecedented success, thus came the movie tie-in...
The first Harry Potter movie came in 2001, after the release of the fourth book, 'The Goblet Of Fire'. The general consensus seemed to be that it was good; as far as films go it was well worth watching. However, it still wasn't a patch on the book, as has been the case with the 7 subsequent films.
Lord of The RingsAnother hugely popular book/movie series, The Lord Of The Rings, experienced a slightly different reception. The books had always been relatively popular for a long time, but there wasn't anywhere near the same level of fandom as the Harry Potter books, until the movie arrived. In the case of Harry Potter, most people would read the book then watch the film, or just watch the film. In the case of Lord of The Rings, people were watching the films, then deciding to read the books.
In the case of Lord of The Rings, the movies received far less criticism, as to many people the films were their first introduction to the series. This could have had a major impact on how the films were received. For example, when a song is covered by a different artist, most people who like the original will hate the cover. Once we have an idea of how something should look or sound, we don't like having to change that idea to a different interpretation.
Your Interpretation Vs The Director'sReading a book allows you to interpret everything however you like. You're given a verbal description of something, and it's down to you to build an idea of how everything looks and sounds in your own head. When a movie comes along you're forced to accept the director's interpretation of those ideas, which can be completely different to yours. The world you have created in your head is destroyed and replaced with someone else's idea which you're forced to accept if you want to watch the film.
Perhaps the books are regularly regarded as being better than the movie simply because the books are read first. It's rare that there is a major book released that is based on a movie, it is generally the other way round. This means that when the book is released, and well received, there is nothing to compare it to. However, when the film is then released it will inevitably be compared with the book, and any areas where it falls short, or anything that is left out, are far more noticeable. The film wouldn't be criticised for these (generally unimportant) issues if it weren't for the expectations placed upon it from the book.