One thing I've seen alluded to in this thread, but not openly stated, is regarding the length of the posts. In the best of the collaborative stories, each post was measured in paragraphs rather than words. The authors took the time and effort to write out some kind of plot development, frequently affecting other characters as well. This is where the challenge came in the made the stories so enjoyable. There just isn't any plot development in "I came in and ordered a Guinness" being your entire post. If you are going to add to the story, then ADD to it - don't just open your mouth for the sake of saying "I was there."

I never did much contributing, being another of the non-writers, but I did enjoy reading many of the threads and watching the storylines unfold. I especially enjoyed when two or more authors would throw in other characters' plot twists, and seeing the lunge-parry-riposte that ensued. JenEllen and Peter T became phenomenal at this. I would sit and read their exchanges slack-jawed in amazement.

I don't know that there is an optimum number of authors, but I've noticed in some of the tavern threads that when there are more than 5 or 6 characters that the stories seem to break down into subgroups, much like a 3d gathering would. It does become somewhat incoherent trying to follow one storyline with too many characters, sometimes due to cross posting, but following each of the smaller stories in the same thread is easier (at least for my simple mind).