I personally downvote opinions that are dangerous. That's it. So if some idiot is giving shitty medical advice because they have no idea what the fuck they are doing and think that vitamin C can cure depression and cancer... DOWNVOTE!
If I don't agree with their opinion, then I will down vote that opinion. The last opinion I down voted was a girl who compared a romantic relationships to a "business transactions". I thought that was pretty disgusting way to look at it so I down voted her because I complete disagree with that statement.