This really gets me.
I haven't spent a whole lot of money of comics. In fact, next to none. I don't have anything against the medium, but I dated a would-be comic artist for about 18 months, and she spent boat-loads on comics. She used to tell me about every major plotline she ever read.
I got interested in many of the characters, but didn't feel the need to spend $70-$80 a month on comics (that was her usual amount).
When some of these comic-book-based movies started appearing, I heard complaints left and right about changes from the original books.
Spidey was bit by a radioactive spider, not a genetically altered spider, etc. For one, I think any medium being adapted to another medium requires changes. And for another thing, comics themselves change continuity themselves, so there isn't a 100% definititive continuity to stick to.