It has been discussed multiple times, and I'm certain this is actually repeating things I have read from a resident sage. Probably in way too many words, though.
Downvotes and comments are completely separate entities. They are not, and should not, be associated! There are way too many holes that are present in this kind of solution, all of which ultimately do not reward explaining the downvote.
In a much more difficult example than what ChrisF provides, suppose someone downvotes and leaves a comment along the lines of "Why are people downvoting this answer?". You can stop complete gibberish, but you can't differentiate otherwise legitimate comments without betraying anonymity.
More problematic is the fact that this system will imply that the presence of a comment about the problems of a post indicates a downvote was cast by that user. We already have problems with this association being made, and revenge downvotes miscast to otherwise helpful and innocent bystanders.
To conclude, there's also a noise production. Sometimes downvoters don't comment because someone else addressed it already. You'll promote people to post "me too" and "Like @dood said, fix your post" comments, which should be flagged and deleted. And under your system, this means they don't get their reputation back, despite explicitly explaining why they downvoted.