2. Because that kind of stuff is banned, the average digg/reddit reader, if he comes across News.YC, finds the content boring and leaves.
3. The custom of the site is to be civil in comments.
4. Votes on comments affect karma.
5. Trolls are fairly rapidly banned. The only reason Giles hasn't already been banned is that I thought perhaps he was joking.
I guess #6. vote weighting will be required soon.
There were a bunch of crappy submissions after we got that big influx of users from TechCrunch (both I and my software agreed about that), but the quality is now back up to where it was before.
I don't think degradation of HN over time is unavoidable, clearly a lot of smart people here put energy into resisting this force of nature.
One challenge is how to preserve the HN code of conduct. I've noticed some misunderstand the voting system as an agree/disagree button. I think the software should notice patterns that don't look right, such as both up- and downvotes on the same comment (popularity contest alert).
Good people + good tools = good HN.
Yesterday we had both a "global warming" story, and a "you can't soak the rich" story. Those kinds of things are far more interesting and accessible to random non hackers, than say, this:
I think between sources like the FT, The Economist, and various economics blogs, I run into a lot of stories that are interesting to me in that same way, that I would never consider submitting here, because they are pretty much guaranteed to degrade into the same old hashed and rehashed debates.
I really think keeping it boring is key: not only keeping the stories boring to your average teenager, but keeping the site looking boring, like a book. :)