When I see a net negative karma post, one of the first things I do is check the comments to see why people are downvoting it. Comments are much better than votes as a signal of the usefulness of a post. Note also that:
I might disagree with the comment, giving me evidence to ignore the downvotes and read the post.
I’m especially interested in reading worthwhile posts with downvotes, because they might contain counterarguments to trendy ideas that people endorse without sufficient scrutiny.
Without comments, downvotes are anonymous. For all I know, the downvoters might have acted after reading a few sentences. Or they might be angry at the poster for personal reasons unrelated to the post. Or they might hold a lot of beliefs that I think are incorrect.
Not sure how the EA Forum algorithm works, but it might be the case that fewer people see a post with downvotes, leading to a feedback loop that can bury a good idea before anyone credible reads it.
In the best case, a comment summarizes the main ideas of the post. Even if the main ideas are clearly wrong, I’d rather hear about them so I can go “ah right, another argument of that form, those tend to be flawed” or “wait a minute, why is that flawed again? Let me think about it.”
At the very least, a comment tells me why the post got downvotes. Without any comments, I have to either (a) blindly trust the downvoters or (b) read some of the (possibly low quality) post.
Comments can save time for everyone else. See (5) and (6).
Comments are easy! I don’t think anyone should downvote without having some reason for downvoting. If you have a reason for downvoting, you can probably spell this reason out with a short comment. This should take a minute or less.
All that being said, I can’t remember any downvoted posts that I enjoyed reading. However, I rarely read downvoted posts because (a) I don’t see many of them and (b) they often have comments.
Cross-posted to LessWrong, although I can’t do a proper cross-post without 100 LessWrong karma.