I'd like to see the intersection of my feed with other people's so that I can see what interests we have in common. - Gabe

A generalization of intersection and union is thresholds: Count(Url) >= N. Then when N is 1, you have union (what you have now on FF); when N is the number of feeds, then you have intersection. - Amit Patel

For example, I'd like to subscribe to a thousand people and then get the URLs that more than 10 of them shared. If you subscribe to infinity people and sort by number of sharings, you might approximating reddit/digg. - Amit Patel