(Downvoted because this is a terrible idea. In my view, the function of a downvote is to improve the value of reading the site by means of cheaply, anonymously communicating the information "Someone would like to see fewer comments like this." If a comment receives multiple downvotes and you're genuinely confused and curious as to why, it's reasonable to politely ask for an explanation, but too many comments of the form Why was this downvoted?---downvoters, explain yourselves just hurts the signal-to-noise ratio. Making a policy of explaining all downvotes would be ridiculous; it would generate so much noise that it might be worse than having no karma system at all.)