Well, to play devil's advocate, I don't have a problem with anonymous downvotes because I think the downvote tooltip explains the purpose well enough. (So do other comments on the post; so do close votes; and so on and so on.) I do have a problem with systematically targeting a particular person with downvotes but that isn't the same problem.

As for the particulars in your suggestion:

for every five downvotes a user leaves without a comment he should lose 5 rep points.

 

If the user explains one downvote in five, no change in rep points occurs.

They already get a rep hit for downvoting which somewhat implies they don't mind taking the rep hit. So... effectively removing an extra point per downvote isn't really much of a deterrent.

On the fith consequetive downvote a visual reminder should appear advising him/her to leave a comment.

Since I don't think downvotes need comments I don't really care about implementing this. Moreover, trying to force people to leave explanations of downvotes is actually a great way to make those people targets for retribution voting. This happens enough as it is and I would rather let people opt out of the drama/argument around whether a particular question or answer deserves a downvote.

The downvoter needn't specify he or she is a downvoter, as long as the comment is constructive and legible.

How are you suggesting enforcing "constructive and legible"?

Any user leaving meaningless incoherent comments or insults ought to be flagged by the community, and have 5 points deducted if he/she refuses to edit their comment.

StackExchange already has guidelines for comments even though most of EL&U ignores the recommended purpose behind comments. Which is to say that you can already flag comments that are incoherent, meaningless, insults or otherwise unhelpful. (I personally don't bother because EL&U seems happy with their comments and I have better battles to fight.)

In the end, I sympathize with your plight but don't really think it needs any drastic attention or policy/feature changes. Chances are high you can outgrow any targeted downvoting by continuing to post valuable content and reaping the benefits thereof.

My two cents.

Well, to play devil's advocate, I don't have a problem with anonymous downvotes because I think the downvote tooltip explains the purpose well enough. (So do other comments on the post; so do close votes; and so on and so on.) I do have a problem with systematically targeting a particular person with downvotes but that isn't the same problem.

As for the particulars in your suggestion:

for every five downvotes a user leaves without a comment he should lose 5 rep points.

If the user explains one downvote in five, no change in rep points occurs.

They already get a rep hit for downvoting which somewhat implies they don't mind taking the rep hit. So... effectively removing an extra point per downvote isn't really much of a deterrent.

On the fith consequetive downvote a visual reminder should appear advising him/her to leave a comment.

Since I don't think downvotes need comments I don't really care about implementing this. Moreover, trying to force people to leave explanations of downvotes is actually a great way to make those people targets for retribution voting. This happens enough as it is and I would rather let people opt out of the drama/argument around whether a particular question or answer deserves a downvote.

The downvoter needn't specify he or she is a downvoter, as long as the comment is constructive and legible.

How are you suggesting enforcing "constructive and legible"?

Any user leaving meaningless incoherent comments or insults ought to be flagged by the community, and have 5 points deducted if he/she refuses to edit their comment.

StackExchange already has guidelines for comments even though most of EL&U ignores the recommended purpose behind comments. Which is to say that you can already flag comments that are incoherent, meaningless, insults or otherwise unhelpful. (I personally don't bother because EL&U seems happy with their comments and I have better battles to fight.)

In the end, I sympathize with your plight but don't really think it needs any drastic attention or policy/feature changes. Chances are high you can outgrow any targeted downvoting by continuing to post valuable content and reaping the benefits thereof.

My two cents.