Personally, I'm also curious as to the rationale when someone downvotes something I've written. I put time and energy into doing it, and I'd like to learn why someone felt it should be downvoted (rather than just left alone). Definitely, if I've make a mistake on technical grounds, I'd like to know what it is.

At the risk of provoking an argument of competing voting systems, something I like at slashdot is that there are descriptive types of votes. (e.g. "interesting", "insightful", "offtopic", "troll", etc.) As a result, there's some guidance as to why someone voted that way.

I agree with not adding a "must post comment" requirement -- that raises the hurdle to vote in the first place. But perhaps there could be a way to annotate a vote, and then to show users not just the total votes on their nodes, but the distribution of annotations. E.g., 10 "insightful" and 2 "technically incorrect". Perhaps a dropdown next to the voting options with various standard comments. (Best with some javascript to change the dropdown to positive or negative based on whether it's ++ or --.)

Another option might be to make a "/msg" textline available right next to the voting options to allow voters to include a quick comment there if they so choose.