If a story has had a significant discussion within about a year, though, we'll kill reposts as dupes. Ditto if the story is off-topic.
The standards are more stringent about people reposting content that they're trying to promote. Deleting and reposting is particularly bad.
I know some people want precise rules, but we're not likely to go there. We want to encourage prudence, not gaming. But we will eventually expand the guidelines and the FAQ to explain more of this stuff. In the meantime, it's best to email hn@ycombinator.com.
As for your feature suggestions, my instinct is against relying on solutions that fragment the community. It's part of HN's DNA to have one community, one front page, one set of posts. The temptation is strong to let it burst at the seams, because there's so much. But I think we're better off finding ways to enhance quality within that constraint, rather than breaking it.