You can comment without any problem. Downvoting is not the main usage of HN, like answering questions is on SO (or commenting, it's a wonder that they still allow any kind of activity for people without karma, too bad the only allowed activity is making questions, that will be incorrectly marked and downvoted most of the time).

I think you need 100 points in order to downvote. HN generally prefers to upvote good content rather than to downvote bad content.

you need >500 points

I've got close to 600 ... yet only see the downvote option on very new posts

It's a mitigation to the "bury brigade" problem digg used to have. I think a relatively low hurdle is a good balance.