Looks pretty nice. With advanced forums we coded it up such that multiple flag types could be added. it works much like how you have it only without the date overlay. The major miss is no display of those bookmarking types.
I think this spec would be a huge enhancement.
Are there any communications or alerts around the due dates? I am nto seeing how that fits in the system. It isn't part of my reading list. It would be good if that date was part of my reading list and was sortable. It would also be good if the my reading list was expandable so that a user could read the post inline and remove it from the list. This would quickly allow them to resolve the reading list.
Other ideas would be a way to auto populate a reading list. So at the forum or discussion level set a auto add new posts to my reading list similar to subscriptions.