First off, before I get critical on Jackson's version of LotR protrayed in teh movies, I thought he did as good of a job as probably anyone could in bringing the story to film. He obviously liked and cared about the books. If you watch any of the extras on the DVDs about the making of the script, you quickly realize how much they agonized over adapting the story for the films. Their dedication to protraying the smallest detail of Middle Earth is beyond all expectations - from elven brooches the hobbits got from Galadriel to Gandalf wearing the red Elven Ring of Power Narya at the end of Return of the King.

That being said, often where Jackson felt he had to alter the story his changes were less than optimal. His changes were often silly Hollywood cliches. screwtype's mention of lacking tension being a factor in there too. Some examples:

1) Jackson altered the nature of the Ring Wraiths' pursuit of the hobbits from the Shire to Rivendell. In the books, the Wraith's were scary and creepy because they were seen but never actually confronted until Weathertop and the Ford of Rivendell. They were seen in the distance, never far way and the tension came from the feeling of being hunted. In the movies, Jackson introducted some additional chase scenes with the hobbits. They were silly and I kept wondering how we were to believe the hobbits managed to escape with them being on foot and the Wraiths on horseback. At Weathertop and Rivendell the hobbits had help to fend off the Wraiths.

2) In The Two Towers, Jackson introduced the warg/orc attack on the road to Helms Deep. Now, the book mentions in passing that wargs were attacking all over Rohan, so to introduce that into the movie I can understand. But to add the whole "fictional" bit with Aragorn falling off the cliff so everyone thinks he's dead is just cheesy and unnecessary - especially when you consider the struggling they had to do to try to trim the story down to a managable size for the movies.

Alas, I've run out of time. I may continue this later (or not ). Anyway, the above gives you a general idea of my thoughts on the movies.