A) My understanding is that "formal culture" is the historical human antidote to trying to interact with large numbers of folks they don't know all that well. Older, more densely populated parts of the globe tend to be more formal than American culture. Yet American culture is the primary influencer of many online communities, including this one. The assumptions made by a less formal culture and the practices which grow out of them start to cause problems when you don't actually know people that well and it simply isn't possible to know everyone here all that well with 100k uniques a day.
B) "Greet people warmly at the door": The general assumption that the ill-mannered newcomers are The Problem tends to promote the problem. Greeting people warmly who are new to the site and speaking with them gives them opportunity and motive to learn the culture and try to fit in. Talking trash about how they are mucking up the place and studiously ignoring them until you are ready to chew them out gives them every reason to behave badly or to assume no one really notices or cares what they do and little opportunity to learn to fit into a polite culture. They don't ever even get to experience the polite culture. All they experience is rejection, insults and such themselves. "Eternal September" isn't because there are new people. It is because the new people don't get inculcated. Hating them on sight and giving them a hard time for simply being new (which is the undercurrent of a lot of posts here) is a major fail if you want to preserve a valued culture. Culture is not preserved by just hanging on to the old folks. It is preserved by teaching it to the new people and helping it grow in a healthy manner.
I'm sure there's more but that's what readily comes to mind and, right this very minute, I'm not up to giving it more thought or time and effort.