Help please: Recommendations for threaded discussion boards

Here's the situation: I am working with a group that would like to do a yearly Bible reading program. Some people will be responsible for getting the discussion/comments started each week, but they anyone to be able to add comments to the discussion.

I am looking, therefore, to create something that basically looks like a blog (or wiki), is as simple and easy as a blog to create posts and allow non-technical users to join in the discussion with their own comments AND sorts the comments into a tree-type threaded discussion display. I am also hoping to find something that is free or relatively cheap (to get rid of ads). I also want it to be able to filter out spam. I have found that many less technical people are not likely to register (even if it is free), so I am thinking using captchas is probably a better option. (But has anyone else noticed how difficult the captchas are to read here on Blogger? Usually takes me at least twice to get the right read...)
Google Groups kind of does what I want, but it doesn't have the graphical appeal of a blog, and it could be a bit more simple to use.
I have also looked at MyFreeForum. It creates very nice forums, but it doesn't really do threading.
Anyone have any good suggestions or can point me to sites where this Bible reading sort of thing is working successfully? Thanks!