What does motivate you to down-vote a comment?
spydez: I down-vote:  * ad hominin attacks with no substance
  * unoriginal snowclone jokes with no substance
  * "+1" and "I agree", if that is the entirety of the post
  * if a post is vastly, enormously over-rated

I try to reserve downvote for the last resort. Well, second to last, now that we have flag.Generally, I ignore the arrows. They are only there for things that make me say "I (dis)agree with this, and it is so good/well thought out/interesting I think everyone on HN should take the time to read it."