I'm not going to say "no spoilers" over a series of books that have been out since the '50s. If for some reason you haven't read all of the Lord of the Rings yet, you probably ought to run out, buy it and read it.
I'm a big fan of The Lord of the Rings, both the original work and the Jackson adaptation. I've also always been interested in the process of adapting novels into movies. The extended version DVD of The Fellowship of the Ring includes a fascinating commentary by Peter Jackson, Frances Walsh and Phillipa Boyens on the process of adapting Volume 1 into a 3 hour film.
The adaptation of Fellowship is one of the most superb adaptations I've ever seen. The spirit of the book is perfectly captured. I don't have an issue over compressing time, reimagining Arwen as an elf warrior, deleting of Tom Bombadil or moving Boromir's death into the first installment of the trilogy. The choices the adaptors made fit perfectly. Frankly, it should have won the Oscar for Best Screenplay Adaptation of 2001 (when you compare it to the rather sloppy adaptation of A Beautiful Mind, it's a crying shame that it didn't win!).
While I like The Two Towers (which is saying quite a lot - I really dislike battle films), I do have major issues with with some of the choices the adaptors made. While many people complain about Faramir (and I will shortly), what particularly bothered me was Arwen agreeing to go to the Grey Havens and making Elrond as something of a villain in the Aragorn-Arwen relationship.
The adaptors worked whole scenes verbatim and some background material from the Appendices into the movies. But they badly misadapted one surprisingly important scene. There is a short scene in the Appendices that really explains why Aragorn and Arwen had such a long, unmarried relationship. Elrond doesn't tell Aragorn to "go away," he tells him that Aragorn should not wed Arwen until he accepts his kingship. That scene puts their relationship in a wholly different context. And the viewer would better understand why they had not wed.
Giving up one's immortality for a mortal life and death is a huge decision. But Arwen's choice sounded definite in the first movie. For Arwen to seemingly obey her father and leave for the Undying Lands was quite frustrating. Now, even though we understand that she will show up, Narsil in hand, in the third movie mitigates against this act slightly, treating Arwen in this way was the biggest mistake of the second movie.
It was particularly frustrating because it was so unnecessary. Eowyn could still be very strongly attracted to Aragorn, and Aragorn could be slightly unsure about Arwen because these things happen completely independently of Arwen. I frankly would have like to have seen Arwen show up hidden among the elf archers since there was a long-standing rumor that she was going to appear at Helm's Deep.
One added scene I believe should be highly regarded as a stunning scene is the scene of Frodo in the ruined city of Osgiliath thinking about putting on the Ring as the Nazgul rises before him. The construction of this scene, from a cinematic standpoint, is quite brilliant. From an adaptation standpoint, the scene is problematic since the destruction of Osgiliath is wholly offscreen and Frodo obviously never goes there. I still love the scene, even though it is out-of-context. But if you're the screenplay adaptor and/or the director, and you're thinking in terms of cinematic punch, and you really want this scene, how do you get Frodo to Osgiliath?
In my humble opinion, it would have been better for The Two Towers if Faramir did not show up until the very end of the movie. His men capture the hobbits, then force Frodo to trick Gollum (a critical plot point). They aren't sure what to do with them. Faramir is somewhere near Osgiliath, that's all they know. Frodo and Sam could still be very worried that the men will take the ring. The men could have marched the hobbits and Gollum to Osgiliath, Frodo could have had his moment on the wall, and Sam could have thrown him down the stairs. And then we meet Faramir. After an hour of Sam and Frodo being sure that whatever man in charge was going to take away the ring, Faramir could have done the right thing and released them. Instead, we had Faramir also being weak and strongly tempted for nearly an hour, and then getting awfully smart in an instant when he sees Frodo and the Nazgul. When Faramir shows up in the third movie, the audience might not be too fond of him...(unless, of course, they've read the book!).
One critical scene that I hope shows up on the extended DVD for The Two Towers is the scene where Frodo and Faramir talk about Boromir. One problem with the movie is characters know things they could not have possibly known without cell phones or some method of instantaneous information transmission. In the book, Faramir explains how he knows about Boromir's death. It's a mystery in the movie.
One scene I wish had been deleted from The Two Towers was "The Fall of Aragorn." The notion of thinking that someone was dead but having them show up alive happens enough in The Lord of the Rings as it is. While I do like the scene of the exhausted Aragorn pushing his way through the doors of the castle, a less melodramatic set-up might have been to simply have Aragorn chase the Wargs too far in the other direction so he has to return to Helm's Deep alone.
I'm very critical of movies that are, essentially, big video games (like, say, anything with "Matrix" in the title). I enjoy movies with special effects integrated into an interesting script with good acting and direction. TTT wavers in this direction a little during some of the Helm's Deep sequence. It's a huge battle sequence with absolutely impossible odds, so I suppose a little of this is understandable. But I found myself comparing the Ent attack on Isengard with the attack Helm's Deep. The screen didn't scream "big video game" at me at all during the Isengard sequence, but it did from time to time during Helm's Deep (particularly any scenes when Orcs were charging the gate or the Rohirrm were riding their horses into battle).
The other thing I disliked about The Two Towers was "Gollum's Song" at the end of the movie. The Enya song at the end of Fellowship was a fine counterpoint to the movie. "Gollum's Song" sounded like bad John Barry music.
Two major scenes, the showdown in Isengard and the meeting with Shelob, have been moved to the third movie. I think that's fine. The conclusion of The Two Towers the movie sets up both events very well.
I know people connected to the third movie have spoken of the adaptation very highly, so I sincerely hope this is the case. The Return of the King is very tricky - it goes from very bleak despair to very great joy, while Frodo emerges as a tragic hero, and Arwen gives up her immortality for love. It carries the gravitas of truly great myth.
Jackson has said that they plan to delete the "Scouring of the Shire," which, while an understandable choice (the "too many climaxes" problem is a bigger problem in movies than in books), is rather too bad. The importance of the "scouring" material is three-fold:
Even if Sauruman is killed off earlier in the movie of The Return of the King than he is in the book (which has been strongly implied by many), his evil influence still could have impacted the Shire. It seems almost too idyllic for the Shire to have been completely unchanged by the War.
I'm not going to argue that "the movie will be ruined" without the "scouring," but that The Return of the King would loose some of its resonance without it.
The third movie in particular is a place where Eowyn can shine. She has several great moments in the third book. I hope they are retained intact in the adaptation.
If the movie ends as the book ends, with Sam, that's a fitting tribute to Tolkein.