The current setup discourages explanations of downvoting, as it makes the downvoter vulnerable to losing karma.
I don’t think so. It’s the very implication that downvoting requires explanation that’s discouraged, since its presence would increase the cost of downvoting. If there is a social norm of explaining your downvotes, or of explaining them when asked to, then downvoting carries an obligation to also write an explaining comment (perhaps on request), which some downvoters won’t be willing to spend effort on (or get involved in), and wouldn’t downvote as a result. But they want to be able to signal disapproval cheaply, hence the comments that appeal to (and so strengthen if approved) the hypothetical social norm of requiring an explanation would also be downvoted.
I don’t think so. It’s the very implication that downvoting requires explanation that’s discouraged, since its presence would increase the cost of downvoting. If there is a social norm of explaining your downvotes, or of explaining them when asked to, then downvoting carries an obligation to also write an explaining comment (perhaps on request), which some downvoters won’t be willing to spend effort on (or get involved in), and wouldn’t downvote as a result. But they want to be able to signal disapproval cheaply, hence the comments that appeal to (and so strengthen if approved) the hypothetical social norm of requiring an explanation would also be downvoted.