I really like the idea of having the main page divided up like so:
That pretty much covers the ranges of use for CC licenses. When there is enough content to fit into these categories, it would be great to switch up to using this.
I think that allowing people to load their own tasks/productions/contest is one great way to support community-building.
I do think that contests create competition which is not necessarily the healthiest way to create community.
I think creating content is two-fold in this type of media sharing site:
1.) contests create high quality content (darwinian-style), but possibly alienate users (non-competitive people who are simply trying to learn about remix, etc) (possibly users hang onto their best tracks until right before the deadline so that others don't copy the best parts???)
2.) collaboration creates more content and more drive to participate (group tasks/production)
Right now ccmixter creates many contest which encourage competition and creation and figuring out ways to encourage users to create their own projects/productions/etc will help to build up the collaboration aspect.
My project Open Clip Art Library (www.openclipart.org) suffers from the reverse, where people are great at collaboration, and making large quantities of files, yet the whole high-quality contest aspect is largely absent (and quality suffers sometimes).
With regards to this collaboration system, I think that mixoff is a cool feature, but it supports more competition and not more non-competitive sharing. I think that an approach similar to sourceforge might even fit better, where projects are setup around some goal, and pools of community members create their own subprojects which create their own goals and rewards. Obviously, this is too large.
However, the idea of someone setting up their own contest or production is enticing because you could get something like where Pat Chilla sets up a production where he is asking for contributions about the color red. Then he says, I am going to take 15 tracks for this disc I'm going to produce and get printed. THen we could setup some type of system where he identifies the goal of the production, in this case, produce and printing the files onto disk. THen, if he fails to do this production, then he gets some type of negative community credit for failing to do this production (similar to slashdot's karma system).
This approach would push cchost to be more like a real social networking site, or even ebay, where if a community member fails, then there are comments and consequences on their profile.
Another type of production/contest/community goal would be someone sets up a task for collecting 2000 sounds of birds under BY-SA license. This would be similar to how Open Clip Art Library works...
Anyway, these are just some ideas off the top of my head...