I would like to suggest the ability to highlight. As a student I want to improve and the only way at this time for me to compare the orignal and the edited revision is to print them out each time. Also it must be frusterating to some mentor when they have to list each correction using 1st paragraph 3rd sentence as the directions to their student of where the correction is needed. Else the mentor just edits the whole thing doing most of the work, hoping the students can see the differnce and learn from it.

Highlighting would allow the students hands on experince correcting their work or even allow them to show their teachers where they are having problems more easily.