I think the "The Lord of the Rings Fanatics Plaza" is okay; just make sure to submit any topics about the books to the "Basic Lore" forum first because the "Advanced Lore" section is by moderator approval only. I haven't made this mistake myself, but I've seen others do it, and oh how embarrassing for them.
http://lotrplaza.com/forum.php
That being said, the only time I ever submitted anything especially rigorous there (an essay outlining what I think the deal was with Sauron's attitude to Morgoth after the First Age, as it happens), no one really cared and only one person replied to it. So I think the amount of discussion there might be a bit inconsistent.
They have no problem with people criticising the films either, as far as I can tell.