I intended* to downvote the comment to express disagreement. It seems pretty standard to vote this way on proposals for changes. The reasons why I disagree with obligatory downvote explanations are

1. It brings heavy asymmetry between downvotes and upvotes, making downvoting much less convenient. A comment which ten people find worth downvoting and two find worth upvoting now stands on -8 (within the most naïve voting model) which shows that it is very probably a bad comment (we already vote more up than down on average). After the suggested change the comment could easily be at zero or positive as the change wouldn't affect the upvoters while most potential downvoters would be lazy to explain.
2. The best strategy to deal with trolls is to downvote without explanation, which your proposal would make impossible.
3. If a comment is stupid for one reason, most downvoters vote down for that single reason, but there is indeed no benefit from stating the same reason n times while the comment sinks down to -n.
4. There is no practical way how to automatically verify the merit of explanations. Therefore it would be easy to game the system, by voting down and posting an empty explanation, or, less explicitly, saying e.g. "the comment is stupid".

The more general idea, i.e. visible names of downvoters, I oppose basically because it introduces a pretty dangerous social dynamic into the forum. There is a good reason why political voting is usually secret and the reason extends beyond politics. Not knowing who voted us down makes us less likely to succumb to the typical failure mode of debating to win and supporting our allies instead of debating to find the truth and supporting good arguments.

Note also that your enumeration is very incomplete, as you can see from my reasons not overlapping much with any possibility you have listed. Furthermore the last one is unnecessarily polarising and practically comprises a false dilemma of only two possibilities - either agreeing with your proposal, or disagreeing with the general idea of accountability.

*) In the end I have retracted my downvote because I wanted to reply to your comment and voting it to -4 would cause me to lose 5 karma points in my reply. So I tried to vote it up to -2, then reply, and after having replied retract my upvote and vote down. Which I haven't managed because you have retracted your comment in between, but still this illustrates one rather bizarre feature of the new anti-trolling "improvement".