In short, a (potential) writer would propose: I'll write this book for a minimum of $X. Interested readers would put into an escrow system whatever that book is worth it to them. If the total escrowed contributions reach $X (presumably by some deadline), the author will write the book and release it for all to read, collecting the escrowed funds. If the kitty doesn't reach $X, or the author doesn't make good, the escrowed funds are returned.
Obviously this generalizes to other media as well.
The only think I don't see this covering well is the speculation on as-yet-unproven writers. Who is going to take a chance on you so you can make your reputation? Maybe that doesn't matter; I think that today, writers, musicians, etc., typically produce their first works on their own to get a publisher interested.