Sorry, but I didn't have time to read any posts but the first one. I'll try to read the others later.
As it comes to permissions, I agree, we should keep most of the forums locked out of sight of guests. Only the non-personal threads should be viewable, but nowhere should a guest be able to post *anywhere* due to spam bots. The only exception I can think of is if you make a small section for guests for 'enquiries' so that guests can ask questions in a small section for would-be members to learn about the site before they have to join.
Otherwise, the threads /viewable/ by guests should be limited. I'm not sure which would be appropriate.. possibly some things in media? Generally as long as the section isn't very personal (ie. some may consider poetry personal) it should be fine to be viewable.