I was listening to your show today when a college professor was seeking advice on responding to student writing. Â Sometimes I write "Powerful Word Choice" or "Creative sentence structure." Â
I was inspired by an instructor at CSU San Marcos, Laurie Stowell, during my masters program.  On my writing she would write comments like "Yes!" and "I agree!"  I felt empowered.  I now try to do the same when I comment on student work.  On their drafts, I'll suggest tips on revising or help with GUMS (Grammar, Usage, Mechanics, and Spelling).  But, on their final draft, when they now feel the most vulnerable . . . I write positive comments to help them feel empowered, just like my professor did for me in college.
You seem to have good experiences both as student and teacher.  But this is just the hardest thing there is.  The same thing you say that works as encouragement or complement to one student can be too cloying or  even insulting to another.  Even your own honest thoughts shared with them can both raise esteem and cause points-off for yourself.  It's a human minefield- no safe methodologies at all.
I had a HS English teacher like Jeannine (and as did Jeannine) who always took the time to add comments, positive and negative. On the other end of the spectrum, some teachers just provided the overall grade at the top of the first page. To their credit, many would remind us that we were free to come in early or after school to discuss the reasons for our grade. But I have the feeling they expected few students would find the time to do that.
While I taught science and math (for some 30 years), I did much the same thing. But with calculations, my "comments" were circling questionable steps and writing a question mark near that circle. Incorrect values were likewise circled, but with a written X. Correct steps were given a check mark, since I always awarded partial credit in proportion to how much was done correctly. A student could get 4/5 points with a wrong answer, if that final answer was just a calculator error.
PLUS (and I know the students appreciated this because many told me) instead of grading with the traditional red ink (a color which carries a lot of psychological baggage), I used green ink.
I can partially agree with RobertB. There's always a potential mine in the field. But I speak from the experience of secondary education, at which point most students have learned to accept praise and criticism, especially when both are used fairly on the same assignment. That may be less true in primary education.
The most memorable comment I ever received on an essay was "Eminently publishable." Â I'm not sure the paper really was so, but the unusual affirmation was powerful indeed.
I typically use phrases such as these for positive feedback: Â "Coherent and cogent" or "Articulate and insightful." Â One advantage of such phrases with younger high school students is that they must now investigate the meanings of these "big" words.
You sure, rjh? Â Joe Biden got in trouble with these compliments to Obama when Obama was wet behind the ears :
"I mean ... articulate and bright and clean and a nice-looking guy..."
"And he's fresh. He's new. He's smart. He's insightful."