A letter game. I like the wikipedia explanation, half roleplaying game and half writing exercise. It's not exactly a traditional letter game, though, because that usually involves two people and I'm hoping for four to six people, which will make the weave a bit more complicated without (hopefully) being so big as to be unmanageable. It will require a bit of flexibility in all the participants. Basically, it differs from a more traditional roleplaying game in that while everyone gets their own character no one is the GM, mod, or whatever (though I do reserve the right to serve as arbitrator and final decision maker if that becomes necessary--I'm hoping that if there are problems, things can be worked out without resorting to the force of 'because I say so'). Everyone has an equal right to contribute major important decisions to the plot, the worldbuilding, and the characterization. It differs from a traditional writing exercise in the number of people I'd like to see involved.
2. Isn't it awfully arrogant of you to have people apply for the privilege of writing with you?
In a way, yes. But I see it this way. I'm applying to you first by putting the letters and the rest of this up here, and if I didn't you probably wouldn't be interested in writing with me. So now it's your turn to make me interested in writing with you. I've tried to give you a head start by thinking about the things I'm interested in in a writing partner and asking about them in the application.
Any other questions are welcome. Comment here, or if you don't want your question to be public, comment in the application post, where comments are screened by default. If you comment there, be sure to leave some way for me to contact you to answer the question.