On the other hand, with a downvote, there's no opportunity for discussion, and the end result is that worthwhile stories about out-of-fashion concepts like Perl or .NET or software patents get suppressed.
It's worth mentioning how often the intent of voting is misconstrued; is it "I disagree with this" or "this adds no value"?
Or "Karma payback time, dude"
In practical terms, it means all of that. What it is "supposed" to mean, I guess, is stuff that PG likes to see (or not)