@Dylan What I'm talking about is metaphysics, mood and culture. How does magic work in the world. What is the mood or general tone, grimdark, noblebright, etc? What are the cultural universals? If this is a single continent then cultures are going to be similar, do they have a more European or a more Eastern flair or are they something more unique. If these three elements are not defined, people's creations won't look like they belong in the same setting together. Cowriting is more than just a bunch of people coming up with ideas. You need that shared skeleton to make sure the ideas mesh with one another and the end product is a believable and immersive setting. 

I'm sorry for my skepticism, but the last one of these I got involved with began with the GM posting a picture of his centaur character which was some erotic, equine abomination straight out of the darkside of furcadia. Defining the theme doesn't restrict creativity, it just makes sure your players are interested in designing a world together and not just in shoehorning or magical realming.