making the identities of downvoters publicly evident to everyone

It sounds to me like censoring. Imagine going to the polls and a bell rings when you cast a ballot against the government. Sort of an Orwellian situation.

It would surely discourage downvoting, but it would undermine the spirit of the voting system as well.

losing XP instead of gaining it for a vote cast to the negative

Why should I be punished for servicing the community? Either you don't understand the voting system or I have a peculiar view of it.

The voting system is a game that encourages you to post high quality contents. If you knew in advance that nobody would downvote your post, you would be tempted of posting the most shabby things that pop out of your head. As the system works now, most people try to post something decent because they fear the community reaction, which is expressed both by downvoting and by pointed answers.

Downvoting, like upvoting, is the feeling of the community. If you perceive these downvotes as being "given in spite", then I am afraid you don't have a clear view of the Monastery yet.

Let's say you got downvoted once. It means one person is telling you that there is something wrong in what you say, e.g. your code is poorly written, or you are suggesting something against the generally accepted rules.

If you get downvoted many times, it means that many people don't agree with you either because your code is wrong, or because you are shouting. If the mass downvoting happen in a thread with a deep level of nested answers (what some people call "a flame") it means that you are debating strongly against the beliefs of many people. It should not surprise you that, in such cases, many people downvote your nodes to express their feelings. That's part of the game as well, IMHO. It helps keeping this site quite more elevated than /.

BTW, I downvoted your proposal, to express my disagreement. Now, what should I do? Wear a "I --ed a node" T-shirt?

Take it easy, pal. It's a game. And very often when a node shows up in PM discussion asking to change the rules, it gets downvoted. The majority of people here like the rules as they are.