I don't mind having the option to click one of a small number of specific tiny phrases describing my downvote: "spam", "offtopic", "bad title", "dupe". But free-form responses? I want to subtly (or unsubtly) correct people, not start up a relationship with them. Indeed, given that a lot of bad posts are made by trolls, a relationship longer than one mouse click is the one thing you don't want to start.

I don't want to waste valuable time figuring out exactly how sure I am that a particular title is bad, or how polite I should be, or whether or not I should take advantage of this high-bandwidth feedback mechanism to start some kind of conversation.

Learn the design lesson of Twitter: More degrees of freedom doesn't necessarily help.