I would think its not that Digg is the pinnacle that these sites can be, but that social news sites are not usually defined by their features-even if a new social news site makes tons of improvements over the digg/reddit model, it simply won't have the user base that digg does. No one wants to use a social news site that no one else uses. Whereas with search, it doesn't really matter that no one else uses it because search isn't about other people.