I really understand that down voting should not be as easy as voting up over a good question/answer but the loss of rep associated with it makes it really something to avoid by a lot of users. I am still learning how to vote up, vote down but more important of all learning to add a proper comment on any down vote given. I know that the moderators comment a lot on users that flag a question / answer that is low quality instead of just down voting, they do that because of ignorance, lack of experience or because they know that down voting will leave them with less rep.
Questions from me here are: How hard would it be to implement a solution where the user down voting would only lose rep (even more than just 1, up to 5 would be acceptable) if he / she did not drop a comment on the down vote? Would that be something interesting to have?
To avoid duplicate reasons for down voting I suggest that if you are down voting in agreement to a previous down vote one you could select a comment that already was made. The same way you get a list of sites that might be duplicate when closing a dupe question a user could choose either to leave their comment on the down vote or chose a comment that already exists that might fit the same reason for the down vote.
P.S. keep in mind this is marked as a discussion, I know that probably there is no valid answer to this (at least not 1 that would satisfy all).