Essentially, I review and use the plus button. It's simple and easy and is always right there when I finish a chapter. I never use the star rating or the like button, can't be bothered. I used to post in the forums quite a bit, but now that's rare - I'm talking about all author forums, not just Mark's. The only exception of late would be Jwolf's forum; I've taken a particular interest in his stories. That will likely fade away as well. I don't have the time to review and post. I prefer to review.
That said, my reviews usually do include comments. What can I say; they just come out. Most of my comments are plot related and the author generally avoids responding to them for the obvious reasons. In any case, I believe plot comments are still useful to the author. It's feedback on where the reader thinks the story's going and even where the reader believes the story has been....if that makes any sense. Guessing at the plot is fun for readers; on that score, my batting average sucks. For newer authors, I usually look for something positive to say about the quality of the writing, character development, flow, pacing and dialogue. I particularly enjoy decent dialogue. However, I don't shy away from offering criticism where I feel it's justified. - "stern but polite" would be how I describe it.