Forums like reddit and hackernews show a total score for every post and comment: Upvotes − Downvotes. This encourages conformism. It encourages people to "read the room". It's even worse then a real room, where you see everyone's reaction to what you say. In this virtual room you only see a final score, the approval or disapproval of the community as a whole.

If up- and downvotes were shown separately, it would encourage unpopular views and it would help people distinguish between comments that express unpopular views (such comments would always have a few upvotes) and comments that are badly formulated, which people wouldn't even upvote if they agree, because they'd be embarrassed by them.