I agree that most authors will take any feedback, positive or negative, but they would prefer ones that explain why the feedback if postive or negative.
Another form of feedback that I personally appreciate is the reader that gives their opinion on what's going on, because it helps me see if I'm getting my "message" across, or just another way of viewing things. A couple of readers that I really appreciate getting emails from live in a quite different culture to me, and their perspective on things is always interesting.
HOWEVER, I also know of at least one author that pulled up and left the internet for a period of time because they received some negative feedback, so that requires clarification.
Negative feedback of a technical nature (eg. typos, story flow, questions on grammar, etc.) are generally well received.
Negative feedback on story content (eg. boring, subject matter not liked, don't believe it's realistic) is more problematic and should be handled carefully, if at all. It's a subjective area and just because the reader doesn't appreciate it, that doesn't mean that there is anything wrong with the story. In these situations, unless a relationship has been built up with the author through previous emails, it's probably better to just leave the story and move on without saying anything
My opinion only, of course.