I like the Stack Overflow system of downvoting

  Downvotes remove 2 reputation from the post owner.
  Downvotes on answers remove 1 reputation from you, the voter.
  Downvotes on questions are free.
  You can vote 30 times per UTC day. You get an additional 10 votes on questions only.

On Reddit, people just tend to downvote when they don't agree with something. That's really it. The only times that I've downvoted posts have been when it's a repost from recent, and on comments I only downvote if it's something that's hateful or doesn't contribute to discussion.