I won't mention any particular failed projects, because the concrete examples aren't that important. The lesson that applied to all of them and which I think is

I won't mention any particular failed projects, because the concrete examples aren't that important. The lesson that applied to all of them and which I think is relevant is this: I relied too much on their being a high-quality audience willing to participate, add quality content, and engage in the conversation. The sad fact is that this doesn't really work: if the project will only work because you have high expectations of the general public, it will inevitably fail. Creating a community like HN that has a reasonably-high quality of participants is incredibly difficult.