I apologise in advance for this long post.
It's actually harder than I thought to rate the performance of the cast; I mean after all, they were only doing what the script said. So in that sense, I suppose their acting was superb. But in actually emulating the real characters in LotR and capturing their 'essence' as best they could despite any disparities between book and script is where the real judgement lies. Like
Boromir, I'll group my thoughts into the categories of Men, Women, Elves, Hobbits, Dwarves and Wizards.
Men:
Aragorn (Viggo Mortensen): A pretty solid performance by Mortensen here. He didn't really capture Aragorn in those rare 'kingly-mode' moments however, as commented already in this thread, and at some points in the movie some of his lines were soap opera-ish. He did well though-
8/10.
Boromir (Sean Bean): Excellent, excellent, excellent. PJ did extremely well here with both the casting and script for Boromir's character. There have been some grievances about little things like his hair colour etc., but Sean Bean really made the character of Boromir something special. He captured Boromir's pride, his fears, his knowledge (and lack of), his love for battle - his everything - superbly:
10/10.
Theoden (Bernard Hill): Another brilliant performance. The transformation of Hill from a weak, doddering man in his dotage to a strong and glorious king was particularly notable. Hill really seemed to fit the character of Theoden and acted it- well.
9.5/10.
Faramir (David Wenham): He acted well for what it was worth. Though he seemed to have the old 'boy trying to win father's love' feel about his character, he did well and put up a strong performance. However, once again, I feel that PJ and that Wenham didn't really bring out his majesty and humility as well as it could've been and that damaged his character. I much prefer the book version-
7.5/10.
Denethor (John Noble): Noble performed brilliantly, despite the fact that the movie didn't really bring out Denethor's character as I would have liked. His expressions were priceless and he had exactly the right
face for the part (weird as it might sound). He played the part of the despairing, yet still cunning, Steward of Gondor impeccably and I really loved the way he said the line in the EE- 'You may triumph for a day....'-
9.5/10.
Eomer (Karl Urban): The character of the Second Marshall of the Mark was really hard done by in the movies, I felt. However, Urban performed well, though he seemed to come across as an overprotective brother and seemed like a common Rohan man, as opposed to a more exceptional one. What really irked me was that Gamling had a greater part in the movies than he did in the book, and that time could have been spent on Eomer, who's sister definitely gets more of the plot. Urban still did well though-
8/10.
Grima (Brad Dourif): Hmm......Dourif played the part of Grima pretty well. It wasn't a stand out performance, but that was probably because his character was overshadowed by the rest of the superb cast in the movies. He suited the character of Grima well though and his acting was a credit to the story;
9/10.
Women (inlcuding female Elves):
Eowyn (Miranda Otto): Otto did very well. I regretted that there weren't more fighting scenes with the character of Eowyn, as did she, but as this is about the perfomance of the actor/actress and not about what the producers did, she had a great outing. She had the 'steelsheen' quality about her and yet showed some of her more gentle sides too. Could have showed Eowyn's unrest a bit better I suppose, but she didn't really have much scope through the script.
9.25/10.
Galadriel (Cate Blanchett): Hard to say really. Blanchett did wonderfully well as PJ's Galadriel, but she didn't quite capture the book version's character. Then again, who can be expected to capture the daughter of Finarfin's character perfectly?

. She did well though- her voice especially amazing;
8.5/10.
Arwen (Liv Tyler): Tyler did well. I didn't like the way she said 'You saw there was a child, you saw my son', nor the 'Come and claim him!' lines, but that wasn't her fault. Her beauty, smooth voice, and wonderful acting more than made up for these occasional lapses by the producers:
9/10.
Elves:
Legolas (Orlando Bloom): Legolas is potrayed very badly in the movies. Nonetheless, Bloom does have a rich voice and to his credit he gave Legolas some respectability. Very graceful-
7/10 for what it's worth.
Elrond (Hugo Weaving): Yes. He acted superbly and was brilliantly cast for Elrond. His slurs on Men were the fault of the scriptwriter/s, but he really carried Elrond's character well-
9.2/10.
Celeborn (Martin Czoskas): Negligable. It's not Czoskas' fault- Celeborn just didn't really have much of a say and didn't seem as wise as he was in the book. I won't rate him, as we didn't really get much of a chance to see his character; or rather what we did see of it wasn't much.
Hobbits:
Frodo (Elijah Wood): I would (get it- Wood?....bad joke) have liked to see and older Frodo, though Wood portrayed him admirably. His pain and suffering, his inner strength and his wisdom were all brought out by Elijah Wood in his role as Frodo. Particularly brilliant in the latter parts of The Return of the King. He had some vestiges of the cliched handsome hero and to some audiences, seemed weak, but his performance was anything but-
9.5/10.
Merry/Pippin (Dominic Monaghan and Billy Boyd): Both wonderful. Light-hearted, funny, spirited, yet also strong and brave were what these two hobbits were. The increasing maturity of Merry and Pippin was really brought out through Monaghan and Boyd, especially with their growth through both physical and emotional battles. Their parting in Meduseld, Merry pledging his sword and begging to be taken into battle to Theoden and Pippin's song in Gondor were especially good. The song especially was a wonderful addition by PJ.
9.5/10.
Sam (Sean Astin): Astin dropped his 'American-ness' to pull off a stunning rendition of Sam in the movie edition of Lord of the Rings. He really was great and once again, was a good example of the examplary casting by the producers. Another
9.5/10 for this sturdy and loyal hobbit.
Dwarves:
Gimli (John Rhys-Davies): Davies was excellent. The comic relief forced upon Gimli's character aside, he really played the part magnificently. He looked exactly like the sturdy Dwarf he is in the books and his gruff manner was harsh, but oddly pleasing. I especially liked him when the Dark Tower fell- his change from jubilation to grief was particularly notable-
8.75/10.
Wizards:
Gandalf (Sir Ian Mckellan): He deserves the 'Sir' title in his name, let me assure you. Along with Sean Bean as Boromir, they were the most brilliantly played characters in the movies. The scene in TTT in Fangorn is particularly moving and wonderful. I won't say anymore, as it's already been said about him, but has definitely earned his
10/10 for me.
Saruman (Christopher Lee): Wow. His deep voice and sudden change from charming to deadly was brilliant. Especially in The Fellowship. Unlike many people, 'The Voice of Saruman' didn't hit it for me. His attempt to charm Gandalf and Co. with his smooth and subtle talk was nowhere as near as good as it was in the book and was really only one or two lines. This is partly attributed to him standing on the top of a very high building, so the voice wouldn't have as great an effect as it does when we picture it in our minds, but he just didn't nail it as I would have liked him to. Strong perfomance though-
9.75/10.
And there endeth my long and boring brief analysis (I know what you're thinking; is this is brief, then you'd hate to see an in-depth analysis

). The movies were brilliant, and overall they get a 9.9/10 for me- I mean, just compare them to other similar movies. Especially fantasy ones. The casting in Lord of the Rings was wonderful and each character brought something to their role. Some were hard done by, but I enjoyed every moment of it.
EDIT: Oh. Guess the post wasn't as long as I thought it was.