What I learned from various social communities:

• Slashdot (I joined 2 years 1 month after it started)
  • The first time that I saw Internet memes take off.
  • Non-democratic front pages are annoying, but they can result in a surprisingly long-lasting community. (TODO: find that article where CmdrTaco points out the benefits of an oligarchy)
  • [quality] Fuck quality. If your comment is one of the first 10, you're golden. Even if you're not, the only important thing is posting quickly.
• Kuro5hin (I joined 1 year 3 months after it started)
  • Excessive navel-gazing is bad.
• Wikipedia (I joined 3 years 9 months after it started)
  • It's actually possible for a community to collectively work towards a single goal, rather than having a nebulous goal of "just hang out and chat".
  • When a community has an actual goal, all of a sudden, professionalism is valued, even though nobody really knows each other.
  • Meritocracy (accomplishment contest) is better than democracy (i.e. popularity contest).
• Digg (I joined 8 months after it started)
  • Democratic front pages are so much faster!
  • Writing catchy headlines is hard.
  • Rewriting a user interface (especially if it's merely to include the technology du jour, AJAX) can totally destroy a site's usuability. (if the Yahoo TV redesign didn't prove this, the Digg redesign certainly did).
  • User communities can do a complete U-turn. (ignore the specifics, focus on the fact that the community has gone through very large changes)
• 4chan (I joined ~3 years after it started)
  • Irreverence is extremely important. Racism, pedophilia, ... things that others consider taboo are exactly what should people lampoon. There are no sacred cows.
  • Know the line between extreme irreverence, and illegality — and get as close to that line as possible.
  • As long as you clearly keep in mind the difference between talking about something and actually doing something, you can talk about any topic, even extremely repulsive ones.
  • It's interesting to stare into the gears of memery for a while, but copypasta gets pretty old pretty fast.
• reddit (I joined 2 years and 1 month after it started)
  • Unembellished front pages help keep the hoi polloi away.
  • Editing after posting allows people to "get the first word in" (Slashdot made it clear how incredibly important this is), while at the same time allowing time to properly copyedit and otherwise refine the comment.
  • All communities die eventually.
  • [quality] SNR is the most important thing.
• Hacker News (I joined 2 years and 1 month after it started)
  • All communities die eventually. No they don't.
  • [quality] People tend to reflexively upmod superficial stories. Counteract this, and look for superficially uninteresting but upon further reflection very deep articles.
• Stack Overflow
  • [quality] It takes time and effort to craft a good response.