Myth #2 - Teachers need to be experts in writing to comment helpfully on papers.
Actually, teachers are experts in their disciplinary fields. You know the disciplinary content and you read disciplinary material all the time. Just because you are an experienced reader in your field, you have more expertise than students do. Moreover, most of you have written in your fields, and that constitutes another level of expertise. Although writing experts can offer advice to help with student writing, you are ultimately much more familiar with the conventions of your disciplines than outsiders can ever be.
If your writing assignments ask students to take on disciplinary content, then you can be comfortable reading as a disciplinary expert. If your writing assignments ask students to take on more general issues or tasks, then your expertise derives from being a highly educated general reader. The key is to think about your expertise as a reader and to respond from that perspective. Every piece of writing should engage its reader. One doesn't need to be an expert in writing to know that a piece of writing is or isn't engaging.
"But, but, but..."
"I know I'd feel more confident in responding to student writing if I knew more about writing."
Granted. So how about a quick refresher of some key principles that might help you feel more confident about the "big picture" of student writing.