This issue have be raised before, but now that Moodle 2.4's Assignments activity has been improved, I want to raise this issue again and ask people to vote for MDL-31355 in the Tracker system.
Here is what I just posted in MDL-31355 .
"I just upgraded to Moodle 2.4, and the new Assignments features made me think about Forums again. Also, I recently spent some time with my university's Blackboard support folks (they are trying to convince me to use Blackboard), and I noticed how disconnected the Calendar is to activities in Blackboard.
The Moodle Forums remain disconnected to the Calendar, and the priority of getting it connected needs to be raised!
In Moodle's 2.4 Assignments, there is now a "Cut-off date". The solution for Forums now becomes clear. The same "Cut-off date" method should be implemented for Forums, and this is how. The Forum's "Due Date" should show in the Calendar. The Forum's "Cut-off Date" should turn the rating feature off (which is what the current Due Date does). I believe that this logic provides a consistent logic with Assignments."