For me one big lesson from metafilter, similarly to HN, is that you cannot create a truly amazing community from self policing and algorithms alone. At least not yet. To build a strong community, incentivizing people to submit and curate good content via upvotes and downvotes is not enough. Hands-on moderation is critical to cultivating a truly great community, because you need truly dedicated users and mods to set the tone, and to quickly step in when a post or comment is really inappropriate. Setting that tone from the very beginning was something metafilter did better than almost any other site. You also need to attract and keep the attention of the top-tier users, those who contribute high-quality content on a frequent basis. And once a few windows are broken, and users start to feel like nobody up top is paying attention, the whole thing starts to break down. Of course, the $5 paywall cannot not be ignored. It's a route I haven't seen many other sites take, and I'm not sure how much revenue it actually generated, but it definitely helped to keep the quality of discourse high. Also, I can hope that someday it turns into a great investment for me when I sell my 4 digit user number.