Hutcheon's Theory of Adaptation

I'll start by saying I hope one day I know as much about a subject as Hutcheon does about adaptation. Hutcheon argues essentially that adaptations are important to any art form because they have a lot to offer, whether it's a different perspective or a more interactive experience. Also in many cases an adaptation will do it's part in keeping the original alive and helping it survive and flourish. He very clearly does not agree with the idea of "fidelity" from one piece to another because he says sometimes an artist is not trying to imitate something but rather trying to improve upon or supplant an existing story or idea. He says on page 20-21 "Perhaps one way to think about unsuccessful adaptations is not in terms of infidelity to a prior text, but in terms of a lack of creativity and skill to make the text one's own and thus autonomous." I think he's absolutely correct and that ties into his earlier statement that statistically speaking some adaptations that are from book to film are extremely successful. That's probably because the directors/producers/writers were just good at what they do, whereas an unsuccessful adaptation may well be the product of less talented people. On page 29 he talks about technology and how because it has provided a lot more mediums for expressing art and stories it has contributed to our idea of fidelity to the imagination. I'm one of the people that have been disappointed by movie adaptations of books that I love and that's because I have many expectations for what I want to see and experience. Also Hutcheon mentions that people are more likely to be critical of an adaptation if it is adapted from a classic. Many people don't want the meaning or significance to be lost. He also makes a really good point about producers creating adaptations because they are more likely to be financially successful and so they're taking less of a risk than they would be if they were making an original tv series or movie. "As George Bluestone pointed out early on, when a film becomes a financial or critical success, the question of its faithfulness is given hardly any thought." Also I just wanted to say that I believe some people's personal likes/dislikes make them more likely to enjoy a film for example rather than a book, or a video game rather than a play, that's just part of technologies impact on people. Back in the day the only option was reading a book or watching a play, whereas now there are many ways to discover a story.


Questions:
Why is it that if something is adapted into the same type of art form it is more offensive than when it is adapted into a different medium? I can't think of any examples but what I mean is if say I were to take the general outline for the Harry Potter books, change a few things, put it into a new setting with different characters but keep things pretty similar and write a book, that would be much worse than if I made a Harry Potter graphic novel.
What makes a particular art form more successful? Film, book, video game etc