A longstanding cliche in film criticism, but often true.  Novels, and similar literary forms, often don't translate well to film.  Many reasons why:

* Film's ability to portray non-realistic worlds (important in many genres, such as SciFi and fantasy) is limited by special effects technology.  The printed word is limited only by the reader's imagination.

* Novels are generally unconstrained in length; films generally have to be under two hours; though studios allow some films to be longer.  More than three-and-a-half hours, though, is almost always a no-no.  When adapting a long novel for film; much gets invariably cut.

* On the opposite side of the coin; feature films generally ''cannot'' be much shorter than 75 minutes.  When turning a short story (such as many fairy tales, anything by DoctorSeuss, etc) into a film; much narrative needs to be ''added'' in order to fit into the film format.  If Disney were to do "authentic" renditions of many fairy tales, for instance (without adding the superfluous material and "cutesy" sidekicks Disney is oft-criticized for), they'd last about twenty minutes.

* Most screenplays are written by someone other than the author of the literary work; in many cases the original author is dead and thus unavailable for consultation.  When the screenwriter takes too much artistic liberty with the original, folks complain.  (On the other hand, when the screenwriter barely changes a thing, folks often complain too).

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''If a picture tells a thousand words, why is it that TheBookWasBetter?''

It all comes down to more opinion then fact.  When it comes to story telling, the best combination is book and picture.  Film tries to accomplish this, but book is still better (in my opinion).  Still frame pictures in books are important, especially if the author has a difficult time describing certain things or events.  Books are unlimited in time and scope, where as film is constrained by time, money, and technology.  For individuals that have a difficult time wading through a long book, or who lack the time to invest in long reading hours, films can indeed be better.  I must agree, however, that from my experience, in large for the most part, the book was better.

One should note, that it is personal imagination that often causes disappointment when a film is seen "after" the book was read.  This is because some individuals have very active imaginations as they create in their mind the scenes that the author describes.  This is especially true of authors that provide minimal (sparsely detailed) descriptions.  Reading a book after viewing a film can prevent one from enjoying the book as much, because the film has already converted description into picture for the reader, thus limiting the scope of one's imagination.

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It's all relative, of course. The Lord of the Rings was arguably better in the book form, but then, for some people it was too dry to finish, so at least for them, the movie was better.

Same thing with Dune. The multi-dimensionality of the books (psychology, subtle intrigue, politics, religion, history, psychic powers, etc) are really just too much to capture on the screen. But I know people who said even the first book was too dry to finish. The screen version on the other hand is at least action-packed and entertaining. :-)

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See also TheMovieWasBetter