Just to summarize points being made in the comments:

PM2Ring makes the point that down votes are not just for the OP.

The primary purpose of votes is to indicate the usefulness of a post to other readers, not just the author.

PM2Ring and ZeroTheHero make the point that giving anonymous vote reasons might actually make users more upset, not less.

PM2Ring

It's generally agreed that constructive comments can often be very helpful to the author of a downvoted post. OTOH, some authors get defensive when their post is criticized, especially if the post is also downvoted, and it's not uncommon for such comment threads to turn ugly, requiring mod intervention. I don't think anonymous comments would improve that situation.

ZeroTheHero

the more comments the greater the amount of debate fodder. If you think attributed comments lead to arguments, wait until you have unattributed ones. The system on balance works well as is, and still serves those interested in improving the contributions.

My own views: The voting process (up or down) is a quick process that is used to indicate the usefulness, quality, clarity, correctness, etc. of a post. So, they are useful to more than just the poster, and on the individual post level matter more than the reputation gained/lost from them, as users can get/lose reputation from other places. Therefore, I believe adding an additional step of somehow indicating why the down vote was cast could deter users from giving down votes, which is something that might already a problem.

Additionally, sometimes in questions that are obviously bad and will be closed, a down vote and a vote to close is sufficient, as the close banner will tell the user how to improve their question.

I agree that down votes without reason isn't always helpful to the OP, but I think adding an additional step of supplying a reason isn't helpful overall.

Just to summarize points being made in the comments:

PM2Ring makes the point that down votes are not just for the OP.

The primary purpose of votes is to indicate the usefulness of a post to other readers, not just the author.

PM2Ring and ZeroTheHero make the point that giving anonymous vote reasons might actually make users more upset, not less.

PM2Ring

It's generally agreed that constructive comments can often be very helpful to the author of a downvoted post. OTOH, some authors get defensive when their post is criticized, especially if the post is also downvoted, and it's not uncommon for such comment threads to turn ugly, requiring mod intervention. I don't think anonymous comments would improve that situation.

ZeroTheHero

the more comments the greater the amount of debate fodder. If you think attributed comments lead to arguments, wait until you have unattributed ones. The system on balance works well as is, and still serves those interested in improving the contributions.

My own views: The voting process (up or down) is a quick process that is used to indicate the usefulness, quality, clarity, correctness, etc. of a post. So, they are useful to more than just the poster, and on the individual post level matter more than the reputation gained/lost from them, as users can get/lose reputation from other places. Therefore, I believe adding an additional step of somehow indicating why the down vote was cast could deter users from giving down votes, which is something that might already a problem.

Additionally, sometimes for questions that are obviously bad and will be closed, a down vote and a vote to close is sufficient, as the close banner will tell the user how to improve their question.

I agree that down votes without reason isn't always helpful to the OP, but I think adding an additional step of supplying a reason isn't helpful overall.