All it takes to get downvote privileges is to submit a few decent stories. Anyone serious about being a solid contributor to the community can get there within a couple of weeks or even days.
People will downvote for whatever reason they want; HN admins can't control that, and couldn't stop people downvoting for disagreement even if they wanted to (though the karma threshold helps to limit downvoters to people who value the health of the community at least to some degree).
The best comments are those that people needn't strongly agree or disagree with, but that cause one to learn something new or think about something in a way they hadn't previously.
If you try and phrase most comments like that (which is surprisingly achievable, even - or especially - on the more contentious topics), downvotes needn't be a problem for you.
I didn't share the site's unstated assumption that making comments that people upvote isn't being a "contributor to the community".
You can control downvoting for disagreement by explicitly stating a norm against it, as StackOverflow does with alt text over the downvote and Reddit does with Reddiquette.
I don't believe I had had a comment downvoted before this one expressing disagreement with downvoters. However, I have found that many downvoted comments do contribute to the discussion, and even earn my upvote about 25% of the time. I don't think the downvote inner circle has a wholly positive effect on the site.
People will downvote for whatever reason they want; HN admins can't control that, and couldn't stop people downvoting for disagreement even if they wanted to (though the karma threshold helps to limit downvoters to people who value the health of the community at least to some degree).
The best comments are those that people needn't strongly agree or disagree with, but that cause one to learn something new or think about something in a way they hadn't previously.
If you try and phrase most comments like that (which is surprisingly achievable, even - or especially - on the more contentious topics), downvotes needn't be a problem for you.