I was thinking a lot about this in relation to Digg last week. Digg succeeds in some categories, but a lot of the stuff that hits the main page is pretty generic. News.yc is an extremely niche community and the content is generally top notch in relation to the general topic (tech startups).

I've been thinking about how you would build a single website that captures these extremely niche communities (similarly to how news.yc captures its demographic) in a way that is better than just having categories (like Digg does).

That would be cool.