Now that people have noticed that reddit's recommendation engine really doesn't work, I'd suggest reddit developers and anyone else interested to take a look at Leonard's thoughts: The Ultra Gleeper: A Recommendation Engine for Web Pages. Leonard's solutions might not be applicable to every recommendation problem out there, but he has given a lot of thought and effort to the issue. His approach avoids (or at least tries really hard to avoid) all the usual pitfalls of such engines.