Well, I've already covered speed and quantity of feedback in online courses, and then I thought about quality of feedback. The more I thought about it, I realized that is kind of tied up with what I wrote about quantity of feedback. It is far better to be spot on with some specific feedback then to ramble on and on and on with thousands of minor errors in a student assignment.
Two important points about quality: (1) you should always find something positive about the student's work, even if it is only the fact that the student actually submitted something; (2) you always add the encouragement that you are more than happy to review a resubmitted assignment. That gives the student the opportunity for improvement and also let's the student know that there was something worthwhile in his/her work that can be improved on.