I don't think that achieves the same thing. First, it requires the user to manually filter out new stuff they aren't interested in, where as a machine learning

I don't think that achieves the same thing. First, it requires the user to manually filter out new stuff they aren't interested in, where as a machine learning approach will evolve as the content space evolves. Think of it like email spam. You can setup manual filters to filter out email spam, but that is a constant and never ending stream of work for you. A simple bayesian filter like pg has described will require far less work and give far better results. In this case, a machine learning approach is even better because it can bring up stories that a user will be very interested in even though the story would never make it to the current homepage.