I thought BROKEBACK was one of the best adaptations I have ever seen--even the descriptive passages in the short story were captured on film almost exactly as I pictured them, which almost never happens.
I don't have a problem with movies being overly faithful to the stories/novels/plays they're adapted from. It isn't a problem that comes up often. The first two HARRY POTTER installments might qualify, though.
Sometimes adaptations go wrong in very small but telling ways. The 1941 film version of THE MALTESE FALCON is an almost line-by-line transcription of the book (which is written in a spare, tough-guy style that reads like a screenplay anyway.) Yet the movie leaves off the final coda (a discussion between Sam Spade and his secretary) which tells us unattractive things about the central character. This is Hollywood meddling, to keep the hero sympathetic.
MYSTIC RIVER is a mediocre book made into a horrific movie. The director, Clint Eastwood, kept the thin, contrived murder-mystery plot but filmed it in a slow, ponderous Important Movie style. At the same time, Eastwood did things like taking out the subtext of Marcia Gay Harden's character--the suggestion that she WANTED her husband found guilty of murder because she was attracted to Sean Penn's character--which gave Harden nothing to play but scaredy-cat. In Eastwood's movies, everyone is two-dimensionnal at most. To be fair, the Lady Macbeth speech by Laura Linney's character is just as stupid and unmotivated in the book.
The worst adaptation I ever saw was the movie version of SOMETHING WICKED THIS WAY COMES. That book is a child's nightmare come to life, full of wild swings between exhilaration and sheer terror. Steven Spielberg was supposed to film it, which would have been perfect--a movie that might have matched the movie that played in my head when I read the book. Instead, it was filmed (by Jack Clayton) in a flat, weak, TV-movie style that made no impression at all.
I ask in all honesty/What would life be?/Without a song and a dance, what are we?/So I say "Thank you for the music/For giving it to me."