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, 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.
You probably should: the Guidelines outright note that the site isn't targeting something like 60% of the population, by the above.
Then maybe you really should. I'd much rather read about off-kilter operating systems and their fans that I would about yet another American centric political article or who got rich this week selling clicks to the highest bidder.
If you'd replace Plan 9 with Linux, this exact sentence could have been uttered in an equivalent forum anytime between 15 and 25 years ago.
If people won't use and promote great but unpopular stuff, things are never going to change, and we'll be stuck with the same crap we have today forever.
One of the things I love about HN is how austere and information-dense the front page is. There are only two stories in the top ten I'm remotely interested in, and I figured that out in about as many seconds.
Tiling window managers are a great tool that I use every day to optimize my work productivity. I like reading how others use them too, so I can learn new stuff from them.
Although I guess one nice thing is that is puts in perspective the fact that stories about tiled window managers and/or Arch Linux and/or God knows what else are always showing up on HN. Kind of like when you see someone, who you can never understand what the hell they're jabbering about, give a talk about something you already know, and you realize that there's no way in hell you would have understood the topic based on what just came out of their mouth. Which is clarifying, because it reveals that they just suck at explaining things, and it's not just that you yourself are a dumbass.