When I was teaching high school English I used to work on writing skills a lot and you are right, positive is so important. I would often say things like “Well your creativity, the most important element that certainly can’t be taught is there in spades! Now let’s just work on that pesky spelling and paragraphing which is the boring bit but needs to be right.” I’d always let them know what was valued and then move on to the specific targets that I wanted them to work on. I think being specific and quantifiable is so important, let them know exactly what you want them to do rather than a vague ‘improve structure’. Good article on an important topic.