If I downvote with comment, it's usually for a fairly specific problem, and usually one that I expect can be addressed if it's pointed out; some very clear logical problem that I can throw a link at, for example, or an isolated offensive statement. I may also comment if the post is problematic for a complicated reason that the poster can't reasonably be expected to figure out, or if its problems are clearly due to ignorance.

Otherwise it's fairly rare for me to do so; I see downvotes as signaling that I don't want to read similar posts, and replying to such a post is likely to generate more posts I don't want to read. This goes double if I think the poster is actually trolling rather than just exhibiting some bias or patch of ignorance. Basically it's a cost-benefit analysis regarding further conversation; if continuing to reply would generate more heat than light, better to just downvote silently and drive on.

It's uncommon for me to receive retaliatory downvotes when I do comment, though.