Canned feedback is not particularly constructive. A generic “you should provide more detail” will never be as helpful as someone explaining “You should add an example of what the flingert did when you decombobulated it.” No matter how verbose a pre-defined comment from a list is, it can’t help but be vague about what the user should do *specifically* to improve their post.

Canned feedback when overused can have the opposite of the desired effect. If every post someone makes keeps getting “not enough detail” down-votes even though the author has tried to improve, the feedback is just going to frustrate them.

In my opinion, this doesn’t solve anything; it is just an indirect way to try to strong-arm a certain type of voter into changing how they vote. A vote is an opinion and it doesn’t need to be justified. 

If this is about "metrics", then we should be gathering information on up-votes as well, not just down-votes. Information about what someone did to earn an up-vote is just as valuable as information about what someone did to deserve a down-vote.