One of the other things that really torqued people off from the movies was when Frodo told Sam to "...go home" after Gollum convinced him that Sam was against them. I had people sittiing next to me saying, in loud whispers: "HE WOULD NEVER SAY THAT! HE WOULD NEVER SAY THAT!!" I was like Man, wave to me on the way down after you jump.
I can't and won't argue the commercial success of the LOTR books, however IMO commercial success is not necessarily the only gauge of literary merit (how well do Shakespeare's collected works sell these days?). To each his own but one of the most noticeable problems with the stories is the meandering plot that goes on and on and on. If you really want to torture yourself read The Silmarillion.
Tolkien's strength comes from his understanding of language and the way he used it to create back story and histories for the peoples of Middle-Earth. I found myself liking the histories in the appendices more than certain aspects of the main story.
Again, don't get me wrong, I did LOVE the books, but I'm not willing to pronounce them untouchable in terms of criticism because *gasp* they're written by Tolkien. I think that the changes Jackson made for the movies made sense in terms of editing and modern audiences.