[bd]

"For example, Hacker News could change its system so that nobody got a point simply for posting a comment."

You don't get a point simply for posting a comment: all points come from upvotes - downvotes. Initial "1 point" label is a purely cosmetic thing.

[cojadate]

Aha, I didn't realise. So HN is already an example of a karma system that doesn't reward hyperactivity.