Imagine if Yahoo! took everyone in your del.icio.us network and showed their tags alongside pages in Yahoo's search results... that's essentially what Google Co-op does. The interesting thing is that organisations such as the Health On The Net Foundation (HON) are encouraged to create profiles and annotate (tag) web sites or pages on particular topics. Thus, when you search for a health-related topic, you see which sites have been labelled, for example, 'Symptoms', 'From Medical Authorities', or 'For Patients' by those authorities to whom you've subscribed.
This should be an useful replacement for all the proposed P2P or server-client systems that would look up each page you visit and display information about the 'authority' rating of that site (when looking for reliable health information, for example), or those sites (such as HON) that propose you use a separate search engine for health information. Seeing as you're most likely to find new pages through the search engine you use every day anyway, it makes sense to display authority ratings alongside search results before you've visited them.
Google Co-op has other features too, such as items that show up at the top of search results pages, created by people to whom you have subscribed, to show more information about your search term. I think this would probably be better if it used OpenSearch, but maybe Google decided that running on-demand queries to multiple sites was too slow.