1. The butchering of Faramir's character. (I hated how Jackson made everyone less of what they were, and yet, somehow, still managed to make a great triloy.)
2. The retardation of Treebeard. (The Ents were slow to speak, but not slow of mind.)
3. The Aragorn/Eoqyn/Arwen love trilogy. (Don't even get me started.)
4. The Redusion of Eomor's character. (Why?)
5. The redusion of Hama's character. (Again, why did they do that? He could have been at the wall like the book, instead of poor Haldir.)
6. How Frodo is much, MUCH less of what he was in the books.
7. How they tried to make Gollum a sympathetic character. I'm sorry, but a guy who murders his own cousin for a piece of gold is not worthy of my sympathies.
8. How they tried to make Grima Wormtongue a sympathetic character.
9. How they made Theoden a stubborn, resentful jerk, instead of the kind soul he was in the books. I'm glad they allowed his true nature to surface in 'Return of the King'.
10. The fact that Frodo held a sword to Sam's throat. Something he, in the books, never even dreamed of doing, even when he was, more or less, the Ring's.