[Lalaith]

1. Major characters given personalities diametrically opposite to how Tolkien created them...eg Tolkien-Theoden "kindly old man" while film-Theoden angry and bitter, in fact playing the role of book-Denethor;
meanwhile Denethor himself - a leader who in the book did his utmost to defend Gondor, ordering the beacons to be lit and the Arrow to summon Rohan: "tall, valiant, more kingly than any man that had appeared in Gondor for many lives of men") turned into a drooling Nero-esque sybarite.

2. Major characters who were not actually transformed were trivialised and made oafish (Gimli, Merry, Pippin) or embarrassingly wet and over-emotional. (Frodo, Eowyn)

3. The stupid Arwen-into-horse scene in TT (didn't actually mind the Arwen-on-horse in FotR)

4. The stupid Treebeard-tricking in TT

5. The stupid Arwen-is-leaving-then-dying thing in TT