I use it only for spam articles. I don't believe I am able to judge what fits the "theme" of the site. As I see it, if you hang out here, you are a "hacker" by some definition. Stories are submitted, and if you find it interesting, you upvote them. If nobody upvotes a story, it is uninteresting to everyone and it dies out in about an hour (how long it takes to leave the first page of the New section). If it is uninteresting to me, I would be inclined to flag it since I don't think it fits my definition of the site, but I restrain myself since other upvotes mean that it is interesting to some (possibly quite large) subset of this community. I can't decide that ahead of time. All I can do is not deprive others of the opportunity to upvote a story before it gets killed.

This is the Hacker News Ideal. Your urge perfectly describes the tension of "interesting to me". Now that I think about it, the upvote/flag seems like a simple, elegant social cognition hack. In order to flag you have to go to the comments page. Positivity is made slightly easier and negativity can always be undone even while it take an extra click.

What's the equivalent for comments?

Thanks. I am a bit more judgmental with comments. If I see a comment where the person is just not getting the point, or seems to be actively trolling, I downvote. I mitigate having to make decisions, by rarely voting, up or down. If I do vote, it's usually to upvote a good comment.

For a community-driven site without a clear statement of intent, 'having a number of upvotes' == 'fits the theme of the site', surely?

Personally I think articles about how to make more money are nothing to do with the hacker mindset, but I wouldn't even consider flagging them if other people found them interesting enough to upvote.

The site started out as 'start-up news'.

The guidelines are pretty clear about this, really:

On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups.

Startup-related stuff is therefore explicitly on topic. Making more money seems on topic re: startups.

Can you define "hacker mindset" for me?

Not really :-)

No better than pointing towards a dusty old copy of the Jargon File -- or the New Hacker's Dictionary, the book version I read when I was a teenager, without which I probably wouldn't be here saying this.

Anyhow, the difference in mindsets between the different readers is EXACTLY why I wouldn't go around flagging things as off-topic that other people liked. Viva la difference.