It would be useful for users to have an easy way to see all assessments completed within a given time period. For example "show all gradeable activities completed during November 2010". This would generate a report of grades either in a single time line or time line grouped by student (probably more useful).
Being able to export the data to Excel would also be handy.
We currently store both time created and time modified in the table grade_grades. We just don't provide a way to report on this data. Time created is probably the more useful field to be able to query against.
This functionality would be useful to determine which users are active for billing or for general course monitoring.