The big difference between metafilter and e.g. Hacker News/reddit is that the moderators at metafilter are very active and often delete posts and comments which are of too low quality, and they have a very active discussion with the community about the moderation (see http://metatalk.metafilter.com/).
In addition to this, signing up (required for posting) costs $5 and this is a significant barrier of entry for creating spam accounts and for users that just don't care enough about quality.
The MeFi moderation team is simply the best at their job that I've ever seen. Considering the shit they have to deal with (which gets way more heated on a daily basis than anything I've seen on Hacker News), the fact that they're able to consistently establish the center and keep MetaFilter welcoming to such a variety of people really astonishes me.
I don't know if the $5 is as significant as the quality of their moderation team. Perhaps it helps regulate the flow of new users. What really wins it for them is that the moderators are willing to respond to each member individually and talk them through the site if need be. I've never seen a Hacker News or a Reddit moderator do anything similar. (The Reddit mods are cool people but moderation isn't their focus at all.)
I think that the $5 fee is important to avoid burning out the moderators with a huge load of work. You can only manage/moderate a limited amount of work, not a tidal wave like noise on the "New" page on Hacker News.
So I think you need both: a high quality team, and a (rough) automated filter to restrain the amount of work put on the team.
To be fair, I can probably count the number of communities that have full-time paid moderators (I mean moderators that actually help guide discussion and interact with the community, not just administrators) on my hands and toes. Being the best isn't hard when you have such little competition.
In addition to this, signing up (required for posting) costs $5 and this is a significant barrier of entry for creating spam accounts and for users that just don't care enough about quality.