I can't say I minded the minor changes made to LOTR. For one thing it's a different medium - some things just won't work the same way. Secondly, much of the stories that inspired Tolkien in the first place comes from a time of oral tradition where stories naturally changes over time in retelling for different audiences. AFAIK, Tolkien hoped to create a work that would fit into this kind of tradition, so in a way it's not a bad thing.
I think a big reason that books not always works so well when done as a film, is that books are typically too long, and a lot has to be cut out of the story in order to make the material fit a 1 hour 40 min. movie. With loss of detail and characters, the story may suffer.
I can't think of a book-to-movie I decidedly hated - I always try to lower my expectations
- but I was
very disappointed by the tacked-on happy ending to Minority Report.