Morgan's progressive mystery exercise reminded me of an effort my old writer's group tried a few years back. There were nine or ten of us, as I recall, and we decided to break up into teams and collaborate on a set of murder mysteries we'd collectively refer to as Enigma. I was the "leader" of Team 3, which meant I had responsibility for keeping the crew moving along in a certain agreed-upon direction. Other than that, we did a lot of winging it, seeing how far we could go with only the most general of plots, and each writer tugging the story in his or her own direction. Each team seemed to have an imbalance, with one or two people doing most of the work. But we'd plug away with our stories, reading chapters to the whole group every other Friday.
It was often a chaotic process for us, but mostly happy chaos. Our team created an insanely bright computer genius who could and did hack into other people's computers, got thrown out of Harvard and sent to prison--and he was the hero. We all had a hard time with our stories, and none of the teams finished. In our case, we had a rogue computer trying to destroy humanity, and we cooked up the idea of its hurling missiles into skyscrapers. We were working through this around 9/11, and the freakish similarity spooked us. We were having a tough time making the story work anyway, and the reality of the World Trade Center was too much for us.
The experience taught us a lesson: Although people have occasionally collaborated successfully, writing is indeed a solitary process. We create our own world and take responsibility for it. We are the masters of that world, for however long it lasts. Just as it's said that no man can serve two masters, no story can serve two masters.