SpankingFire:
Simply, do you want critical comments if they are worded politely in order to grow as an author or would you rather get a word of thanks?
Speaking for myself as an author, the answer is "hell yes I want critical comments". Being human, I will also gladly accept worshipful praise, but an informed tip about how to improve my work is a wonderfully useful thing.
As a commenter, I not only try to be polite when offering such a comment, but I try to balance it by pointing out something I liked about the story.