Far be it from me to put forth a logical fallacy (which guarantees that what follows will be exactly that, I'm well aware), but can you point out a forum that implements a voting system which requires some form of comment?
You have forums that support a reputation system with comments, and forums that offer only non-anonymous voting, but I've yet to see one that has both a reputation system (or a commenting system) and a non-anonymous voting system. In fact, voting from what I've seen has been overwhelmingly anonymous.
I can sort of understand the curiosity involved with wanting to know why someone has downvoted a post, but believe that the anonymous system works in everyone's best interest. Where reputational voting is concerned, of course, anonymity is removed.
I would not like anonymous comments. I think if you have something to say then you should be willing to have your name attached to it especially when what you have to say affects the reputation of another user. Having said that I have seen how anonymous voting can be abused. We have seen users whose posting quality has tanked because another user had gone through the forums and downvoted averything by that user. Something that could alleviate that would be to disallow multiple downvotes within a given time frame, especially against a particular user. However, this would not prevent someone from downvoting one post. I'm assuming that this arose from the regex thread in the vb.net forum. In that case the user who downvoted had three (actually four) options:
downvote with comment
The first choice gives you a hit on your posting quality whereas the second choice affects your reputation. As you saw in the thread, I chose option 3. I would have chosen option 4 had it not been for your reaction to the downvote (which I will state again that I was not responsible for). While I found your proposed solution overly complex for the problem, the OP was free to ignore it.
We have seen users whose posting quality has tanked because another user had gone through the forums and downvoted averything by that user.
The algorithm to calculate post quality score was fixed to make this not a large concern. Actually, the algorithm was much more forgiving originally in regards to this specific scenerio. When we switched off of the vBulletin platform, a new algorithm was created. After finding the new algorithm was subject to abuse, we reverted to the original algorithm we had been using.