I think a good system would be to have just two voting axes:

1) "worth reading" / "not worth reading" 2) agree / disagree

Then add a system of whatever flags you think are necessary, like "spam".

I treat the current voting arrows as if they meant (1). I vote up things that I think are worth other people's time to read; vote down things that are a waste of time to read. I very rarely downvote. In some cases where others might more liberally downvote, I simply don't vote.