Lord of the Rings. That's right, the whole thing. I could spend literally hours telling you each place in the story that he made unnecessary changes. I'm not the kind of fan who thinks a film made from a book should copy everything in the book down to the last exclamation point. I understand the distinction between the written word and a story told through a visual medium. I made a lot of allowances for changes made for the sake of continuity, time, even the injection of little bits of humor or extensions of things that were not so prominent in the book, as well as omissions of characters that were in sequences that flowed outside the main storyline. I even could see where including the Scouring of the Shire would have made too much movie for the theatrical release. And even with all that, he couldn't keep his hands off the rest. He made noble characters ignoble, made proud people silly, created enmity between friendly nations, added melodramatic conflict where there was already reasonable conflict, out-and-out made things up, replaced characters with others, turned Frodo and Sam into soap opera characters, and killed my favorite moments one by one. The more my husband and I reread the book and watch the movies, the worse they get, so that to good about them isn't enough to overcome the bad. He did some pretty awesome work with costumes, sets and scenery, casting, and of course the incredible music by Howard Shore, and I just can't sit through them without Rifftrax anymore.
Also, Harry Potter and the Half-Blood Prince has been a sweeping disappointment... I was actually okay with it since I didn't read the book recently. But my tween daughter was practically yelling at the movie screen. "The Burrow never got burned down!! !" "Where's Bill?" "Snape gave a whole speech just there! Where is it?"
There's just no theatrical disappointment like a movie based upon a book you love. Unless it's X3. That was sucktastic. Or Spider-Man 3. Hoo.
Now I'm bummed. 
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"Pack up my head, I'm goin' to Paris!" - P.W.
The world loves diversity... as long as it's pretty, makes them look smart and doesn't put them out in any way.
There's the road, and the road less traveled, and then there's MY road.