IMO an improvement would be effected if it was not possible to downvote a node without offering a reply to it.

I disagree. All you would achieve is a skewed, biased voting behaviour: people just wouldn't bother downvoting on a node any more if they have to comment on it. And you'd get more nodes with a higher rep, and the associated higher XP level of their respective authors, as a result. Undeservedly so, if I may say so.