The difficulty (that no-one seems to have figured out how to solve) is a system that effectively hides low quality posts without becoming more of an echo chamber over time. While a community is small it is not too much of a problem, because even mildly down voted posts have good attention—but as it grows, highly up voted posts that reflect existing tastes or confirm existing biases increasingly dominate.
The difficulty (that no-one seems to have figured out how to solve) is a system that effectively hides low quality posts without becoming more of an echo chamber over time. While a community is small it is not too much of a problem, because even mildly down voted posts have good attention—but as it grows, highly up voted posts that reflect existing tastes or confirm existing biases increasingly dominate.