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.