I think it's important to realize why The End was successful: one very talented person was in charge of writing everything. In my experience, true collaborative fiction goes one of two ways:
1. Everybody writes about the same characters, causing some characters' behaviors and motivations to vary from author to author and post to post, creating a lot of toe-stepping and foiled intentions.
2. Everybody creates their own characters or subjects and writes exclusively about those, creating a wide narrative that never progresses deeply as no author feels comfortable advancing the timeline, for fear of ruining what someone else had in mind.
I hate to say it, but this sort of things needs a firm structure, or at least a very good understanding of everybody's role and the scope of each writer's responsibility.
If it were up to me, I'd propose a small (no more than three people) writing team for the primary canon (this being influenced by the vote/discussion structure, if it's in place), with other contributors being able to submit side stories, art, music, animation... whatever would be entertaining.
Also, the writers should have a decent idea what the setting is and how far any fantastic elements can go before they begin, so that the story doesn't turn into Lost, which as far as I've heard is now about zombies, time travel, and Hitler.