Have we mentioned Eragon? I don't have many specifics because I was so disgusted by the movie that I didn't retain much. I'll admit the book was entirely LotR-lite, but at least it stayed relatively true to it's source material. I... I honestly don't know what the people who made the movies read.
How does one ruin Eragon? So utterly derivative and self-important, while wearing the guise of originality. If you have orcs in your book, call them "orcs". At least the Forgotten Realms/Dragonlance/whatever novels don't beat around the bush in that regard. They knew they were ridiculously derivative of LotR, Warhammer and, of course, D&D, and used such things to their advantage. They even managed humour, tension, unpredictability and likable characters in some cases.
Eragon is essentially a Mary-Sue-athon where the author looked at all the above sources and missed the point, themes and cool factor entirely. LotR is great because it's all about defying fate, not altering reality so that you become genetically superior (in which Eragon carries more racist undertones than LotR ever could).
I guess I am being very very harsh and it's not like I'm making judgements about those who read the book, but there aren't many ways you can essentially ruin Eragon, since it's already a mockery of the already rather poor swords and sorcery fantasy genre.