A quote from one of the moderators:
>Intellectual curiosity is everywhere, and it’s present in all demographics,
>We want Hacker News to grow in all demographics, because there’s just intellectually interesting contributions from all of those communities—a greater diversity of content, of conversations, of topics, et cetera.
Which is just another way of stating the guidelines:
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.
Off-Topic: Most stories about politics, or crime, or sports, or celebrities, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic.
The stuff you don't like is already off-topic, unless there is intellectually interesting discussion to be had.