They might have changed it recently, but when I was active on SO, it was -1/-1 for downvoting, 0/+10 for upvoting. The idea was, iirc, that high-rep users have typically been active quite a while, contributed a lot of value and aren't troublemakers (or moderators would have confronted them), so they are trust-worthy to not abuse downvoting (while they could afford it). Low-rep users on the other hand can't afford to abuse downvotes (and it would take ten downvotes to undo one upvote) and have every incentive to only downvote when they do believe it's a bad answer. On the topics I was active, it was also very common to explain downvotes and flags so the users can improve their questions or answers (and downvotes would typically be removed if that happened).