Much of the Web's infrastructure is based on rating systems. When you see an eBay vendor with a positive rating, for example, it means she has honored transactions in the past. More importantly, her positive rating is something she won't want to risk in the future by messing up *your* transaction. Whether it's a "Score 5: funny" rating on Slashdot or accumulating business connections on LinkedIn, building up one's rating is a metagame that many online businesses rely on.
Yet ratings aren't for every website. Many early search interfaces (some still can be found today) indicated match confidence with a numeric rating. But what is one to make of this "confidence?" Either a search result meets a user's need, or it doesn't. Users are more satisfied when they make the judgment themselves, based on link titles and descriptions. The only necessary search confidence rating is the order of search results, best matches at the top.
It's common for users to rate websites -- at Digg, they do nothing but. One Digg ancestor was Web ratings system Wisewire. Developed to compete with the expert-edited Yahoo Directory, Wisewire's system was simple and fast. Users visited sites from an automatically generated topic list, and voted them good or bad. Ratings would build up and the best sites climbed to the top, while bad ones dropped off.
Simple, however, is not always best. Wisewire's limited feedback channel led to usability issues. Users wondered, "How can I give this site a vote? It addressed my goals, but I can't say if it will help the next person." In addition, anonymity meant users had no metagame to play. They received no direct benefit from voting.
On Digg, users have identities and post comments on their votes -- then their comments are themselves "digged" up or down. So Digg doesn't just rate websites, it rates users as well, giving them a metagame incentive to participate.