|Perl Monk, Perl Meditation|
i think that having a "comments" section for -- that could be send only to the author of the node or posted for public consumption is a really sound idea...
downvoting someone without taking the time to say why, IMO, provides an arena for spite (as in the case of someone using up their daily votes to downvote multiple (often unrelated) nodes from a single PM) or silent criticism... and maybe other things...
downvoting should be meant to assit another in understanding why something is --, to, as cajun remarks, help them do it better next time... otherwise, one can end up asking for the wisdom of Perl Monks and getting only scorn or flipness...
having said that, i have to agree that having only ++ is a very good idea... why spend time -- something when there is much to ++...
besides if you have something negative to say, you can simply /msg that PM and they'll get it next time they log on...