Every professor writes. Most teachers recognize good or bad writing when they see it. What they may lack are the skills of analysis and diagnosis that would help them to identify students' underlying problems as well as skills in providing correction, feedback, and guidance that will enable students to improve. If one asks students to write more, how can one conscientiously comment on that writing without an impossible increase in time spent reading and grading papers. To help students know what to look for, provide models - both of well written papers on a given topic as well as of papers with your own comments about some common problems.
Short papers are more valualbe than long papers if they provide a stimulus to students to write.
Form sub-groups of four or five students and have each group read and comment on each others papers. Have students hand in more than one draft. Requiring resubmission encourages the student to identify and correct problems emerging in earlier drafts. Up to a point, specific comments lead to greater learning. Simply noting errors is not helpful if the student doesn't know how to correct the errors.