Threads like the above are one of many that make it clear that the anonymous nature of down-voting sometimes causes a heated discussion as to why a down-vote was made. This is understandable, in some cases, were the member providing the answer has been down voted on what they deem as a perfectly appropriate answer. They then feel they at least deserve an explanation as to the reasons why they're reputation has been damaged.
Just as different actions (correct answers, up-votes, committed edits etc.) carry various weight in terms of the amount of rep given/taken - would it not be possible to implement the same with regards to down-voting? For example, I may want to remain anonymous but -1 somebody, which is fine - but then if I wanted to -5 their answer then I could only do so by providing an explanation as that particular 'type' of down-vote requires an explanation. It will also reduce my reputation by the same.