I was looking over my notes for a test this Tuesday, and when feedback came up, it reminded me of something my professor said. He asked us to imagine we were being approached by the principle/superintendent who asked how to best raise the performance of our students. He said the obvious answer would be to make the class size smaller, which would allow us to be able to focus on more students and their work.
The likelihood that decreasing class sizes will actually be possible is fairly low, though. However, the professor explained that he had a trick that would raise the performance of the students even more than smaller classes. If someone received a paper back with a red "D" on it, they would simply be getting evaluative feedback. That feedback does not give the student anything to build off of. Unaware of what they did wrong or how they could do it better, the student has no real direction given to them.
However, descriptive feedback is much more helpful to the student. This type of feedback lets the students know how well they're doing or how to improve. Whenever a teacher marks an answer wrong without telling the student why, that isn't really doing the student any favors. If the teacher marks the answer wrong, though, and explains to the student why it was wrong, that observation can allow the student to build upon a foundation that was not there before. Obviously, this is going to take more time and effort on the teachers part, but why else would someone teach if not to help out students in the best way they can?