Just some thoughts: putting the full message in the discussion tree means that people will tend to write shorter / comment-like messages, and not in-depth argumentations.

On the other end, having only a tree of authors is a bit to slim, cause you have to read the content of each message to know what's going on. I think people who reply to a thread should give it a "reply title". You know it's the same thread, but you can have an idea of the content with the reply title.