Ask HN: What features of HN unlock with karma? Are there user levels?

The set of features and thresholds change from time to time, often without notice. I'm not sure that this are the current numbers.

flag posts at 20 points

create a poll at 200 points

change your top bar color at 200 points

downvote a comment at 500 points

I think there's an advanced level that lets you downvote comments, though I do find - particularly of late - it's pedantic and politically abused rather than sensible content moderation.