Bowing to very vocal demand, we've added a new feature to the workflow system: the ability to add a note to a workflow item which you don't currently own. (It continues to be the case that the current owner is the only person who can change the workflow item in any way.)
The ability to add these notes is controlled by the workflow definition. This switch is turned off for all existing workflows. If you want to take advantage of this new feature, you need to edit your existing workflow definitions and turn the switch on.
After doing so, anyone who's not the current owner of a workflow item is able to add a note to it using the new box at the bottom of the item overview page.
