The goals for these types of discussions are clearly connected to students' own writing. You are showing them how concepts discussed in class translate into a particular type of writing. Or you are determining whether a writing sample meets the criteria for an assignment. During these discussions, you'll want to guide students with questions like, "What's effective about this piece of writing?" But don't hesitate to point out the problems areas in the sample.
Discussions about writing should be student-centered, but you also need to provide clear judgments. If an essay has some serious problems, be sure students are aware of this when they leave. When planning, highlight places where an essay is effective or ineffective. If students do not raise the same concerns, point these out for them. Your goal for these discussions is to have students walk away with a greater sense of what to focus on and what to avoid in their own writing.