On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.
This guideline applied when this site was populated mostly by hacker entrepreneurs. There have been polls in the past that revealed the HN userbase is now comprised of mostly "non-hackers." As this site has become more popular it stands to reason that we're in the minority now as there simply aren't that many people who identify as hackers (let alone entrepreneurs).
If you reduce it further to "anything that gratifies one's intellectual curiosity" then you open the gates to anything. That is why we have stories about smiling in Russia on the front page.
