I tend to downvote posts complaining about being downvoted, not really to counteract "sympathy upvoting", just because it doesn't bring anything to the conversation.

It's true that there should be a way to get information as to "why a post was downvoted", and the only way to do so is to ask, which comes off as "complaining". I don't really see a good solution for this, maybe slashdot-like tags would be better.