The problem with the LOTR movies isn't that they got some details wrong - Aragorn giving the hobbits the barrow swords instead of Bombadil doing it isn't important, for example. It isn't even that they changed character dynamics (although Comic Relief Gimli really grates). The problem is that they didn't do so well on the overarching themes of the book - the contrast between Boromir (who loved war and glory for their own sake) and Faramir (who fought just as eagerly, but for no reason other than to preserve his beloved home) is totally lost, for example.