The traditional story will move from beginning to middle and finally to end but in the case of many or most collaborative fiction stories at Pan Historia the idea is never really to end. When it comes to the storyline I prefer to think of my stories more in the light of a serial or ongoing series. There may be story conclusions to individual stories but the characters need new challenges to crop up all the time.
This is where collaborative writing needs more than one head because, frankly, coming up with new plot ideas all the time to keep a train moving indefinitely into the future can be daunting. Some days you just don't have the inspiration and bouncing ideas off the other writing partners can be a great thing.
I tend to assume that most of the readers of my fiction are writers in the same story (though I have found with great pleasure, on occasion, that there are other readers who enjoy the stories) and so I always have a little line drawn in the sand - how much to reveal and how much to conceal so that they will still have the joy of surprise over plot twists. This is a true balancing act for me though I understand different writers resolve this in different ways.
There are two ways to deal with writing discussions - one through private messages between two writers and the other through forum or chat room discussions between groups of writers. My problem with the chat room is that if you need everyone there you might be lacking a window of opportunity while bulletin boards allow for everyone's unique time table. Generally I use the private messages over public discussions because of my desire to give everyone else the thrill of surprise, but eventually you will come to a place where everyone needs to be on board. However writers prefer to deal with plot discussions Pan Historia has the tools for it: forum boards available only to the group (novel as we say in Panspeak); chat room; or instant messaging onsite. With the recent re-enactment of the "Gunfight at the OK Corral" by my Tombstone writing group we used all of the available media. We posted widely on the forums in the weeks leading up to the writing event, then we instant messaged each other to work details between characters, and finally on the day we kept coordinated using the chat room.
Which leads me to an interesting aside: as far as I know most collaborative fiction sites are using models adapted for other kinds of interactive social media and Pan Historia is one of the few built specifically to cater to role-playing collaborative writers. I think once people try the site and get past the first "oh shit I don't know where to find anything or what does this button do" feelings they should find themselves in a complete environment catering almost exclusively to their interests.
Another important thing to remember when discussing plot with your fellow writers is to remember to listen and to be flexible. You might have something in mind and it becomes your 'darling' and you want to control that plot, but don't. You'll end up writing alone - which is fine if you're a novelist but kind of dull on in a collaborative situation. Also, and this has happened to me frequently, if you let someone else's ideas help to shape yours you might just find that the resulting story is even better than the one you originally imagined.