First and foremost, student writing is improving by leaps and bounds. When I read their blogs (which, by the way, are mature, insightful, funny and engaging), I don't find myself pulling my hair out over the careless mistakes they make in formal papers. Not every post is perfect, but the majority are well written and free of grammar and usage issues that I am so familiar with seeing in their other work. If they become sloppy, all I need to do is politely comment about it on their blog, and I don't see it again.
Via Nik Peachey