The ability to downvote gets unlocked when you get enough karma. There are other abilities (e.g. creating a poll, flagging posts). I don't know whether there's

The ability to downvote gets unlocked when you get enough karma. There are other abilities (e.g. creating a poll, flagging posts). I don't know whether there's a definitive list with thresholds anywhere (possibly in the Arc source code to HN).