J.K. Rowling demonstrates an important fact: Good writing has its own appeal. The Harry Potter series was well-written with simple concepts, good action descriptions, and relatively easy motivations. It translated well to the screen. (As proof of that fact, Ms. Rowling now has more money from royalties on her work than the British Royal Family had when Elizabeth II was still queen.)
By contrast, the Frank Herbert series starting with Dune did NOT translate well to the screen because so much of what happened was "stream of consciousness" (voice-over as opposed to dialog). A lot of the history you needed to understand the machinations of the Bene Gesserit group required an "expository lump" - i.e. you needed a history lesson to put it all together while watching the movie. Whereas the book had the history lesson interspersed with the action and dialog. Didn't translate from print to screen so well.
Another example of the "translation" issue was the D.F. Jones novel Colossus, which was made into the movie Colossus: The Forbin Project. There, the novel translated well for the most part and was an enjoyable, if chilling, story.
