There is an interesting discussion about the usability of the Moodle 3.1 grading interface going on int he Assignment forum.
As a student who has taken many online courses in Moodle as a graduate student, I completely understand that the majority of asynchronous coursework will be document based, but even when turning in links to something like a Google Doc, or a website, the auto-pdf viewer pane is just completely out of place. The dynamic editing/collaboration ability through Google Docs is far better than just a one way annotation.
I wonder if there would be a way for developers to turn that pain into a web-browser (the way Doctopus and Goobric work) to really put everything in once place if the submission was in fact a link.
It has potential for sure, but there is some design work that needs to be done .