This is an interesting question, and the answer isn't easy.  Flac+mp3 will please a large number of people, but it's a big upload.  Ogg is a nice middle ground.  It's an open format, which goes over well here, and the quality is better than mp3.

My biggest fear with that is that in order to keep things easy, we need to able to be at least somewhat arbitrary.  The issue there is that someone will eventually get offended that we used varying standards to apply a rating, but to some extent you need to hold on to that level of agility.  Take Wikipedia, for instance.  Their notability standards are so utterly convoluted now that they can be interpreted any way people want to, which ultimately leads to the same problem they were intended to prevent.  Keeping the standards simple but a little bit open to interpretation (and being willing to consider changing a rating if someone has a good objection) might be the best way to go.

I'm not familiar with .ugh.  What modeler uses it?

Honestly, the way I prefer to "crowdsource" on OGA is to *let* people submit stuff freely if I haven't officially given someone the job, but also offer to officially guarantee the job to someone who applies for it (assuming I like their portfolio).  That gives me the best of both worlds -- I can "crowdsource" in a way that lets people casually submit things without applying, but also give artists a guarantee that they'll get paid if they're interested in taking the time to apply.