Mainly SEO spam with text copied from other sites and lots of ads/referral links to make the owner a profit. But after thinking about it more, those would be rather easy to filter based on ad code in the HTML.

A much bigger challenge will be the ranking of search results. When searching for a term and there are 100 pages in the index that contain it, which of these pages should be shown first? Google developed the Pagerank when they started out, so that might be a good starting point to research further.

This sounds like a very interesting idea. I agree that Yacy doesnt work, when I checked it out years ago it was a completely bloated mess. Not sure how viable how your idea is, because Im not familiar with webrings, and not sure how the federation will work. Anyway the main challenge for this project will be to actually give useful search results, both early on when there are very few crawlers, and also later once spammers try to abuse it.