1. At the moment it obscures the submission pane. We definitely want to improve this usability. Originally we were unable to allow users to adjust the size of the columns, since our last release this has changed and we are considering improving this.
2. We are adding that to our December 2013 release of Joule so that change would be included.
3. Currently only Forums is supported and we could probably add rating capability to the interface, but the original idea was that you would review the forum and rate the posts then use grading interface to grade the overall performance. The idea was that ratings would be used as a metric for grading but not as the grade. Database and glossary don't support grading. Long term I would love to convince Moodle HQ to add grading to many more activities.
4. At the moment no, but that is something we will be looking into as part of the Assignment grading work flow that was added in Moodle 2.6. Long term imo this interface needs to support all grading options a teacher would want for a course.
5. Turnitin would depend on what plugin you are using. Joule grader will have an API that activity plugins can take advantage of. Turnitin Direct would need to add Joule grader support to their plugin. If you are using the plagiarism plugin not the activity then that like PDF annotation should be supported in this interface. But needs to be added. This is something we would need to look into further.