Why ruin the book with the movie?

The latest craze is making film adaptations of books.  While these book adaptations have been going on forever, with adaptations of Dracula and Sherlock Holmes at the very turn of the century, more and more frequently the fodder for these films has been young adult literature and fantasy films.  Advanced technology and CGI has made it easier to shoot these films with their fantastic sequences, and as these movies gain popularity, more authors try to piggyback on their success, creating an unending cycle.  The book I previously reviewed, Divergent, is one such work, creating a dystopian universe to follow the success of The Hunger Games, and film producers, with the same idea in mind, jumped on and began producing the series.

In order for a book-to-movie adaptation to be successful and popular, a fair number of criteria must be met. 

• It needs to appeal to its original audience, those very first readers, as well as people who have not yet read the book.  Better yet, it should inspire people to read that original book.  It cannot be just a fan project, made by fans for fans.  Even the Harry Potter books could not have succeeded if they did not appeal to the parents, grandparents, brothers and sisters of those fans who read the book the first time.
• It has to please the fans. That is so hard to do, because each reader creates their own vision of what their world looks like.  I envision a character as blonde with blue eyes, my sister sees him as redheaded with glasses, and the film picks a short brunette with a crooked nose.  
• It has to be new.  Only so many boy wizard movies can ever be published after Harry Potter, unless they are reboots.  Each thematic idea can only be replicated so many times, and film audiences know that as they look for something new.
• It has to be very selective in what it includes and leaves out.  I think Lord of the Rings did a wonderful job of making those choices, but they left out a scene with important character development at the end of the novel, the Scouring of the Shire.  Personally it was an issue for me, but through trimming the thousand page novel into 3 films, The Lord of the Rings made many of the right choices in what to cut, what to change, and what to add to make the films widely appealing.

So go ahead, make your movies out of the books.  Just know that the fans are out there, watching and waiting.  Your next hit won't be as big as the last one, because audiences get bored and the fans will judge you fiercely.  But as long as there's money to be made, and someone to watch it, the red carpet will keep rolling