The high quality subs are high quality because they have limited appeal. It's the same way HN keeps a reasonable level of quality, if there way an influx of people here who weren't interested in technology/programming and just wanted to talk about video games or politics then the quality would go down.
askhistorians and askscience have relatively broad appeal, have a very high level of quality and are extremely heavily moderated. In my experince (and it is extensive, I signed up in august 2006) the heavier you moderate a subreddit the better the quality, although a bad moderator can kill a subreddit fast.
> the heavier you moderate a subreddit the better the quality
Also focus. AH and AS are both focused on what they do which allows them to have pretty clear rules, that doesn't work for all subs. And bigger subs with subjective rules can easily end in revolting users and sinking quality.
Avoiding a sort of Web 2.0 of the old usenet "Back to School newbie flood" certainly helps. It takes a really good mod team to stand up against the flood. See the decline of /r/books, for instance.
