a personal organizer should certainly give back information on how much work has been done lately, to "reward"/confirm to the user that he actually did something lately. Should use bars and graphs.
This should take a big UI space (another window or replace the tree pane), since it's is important to have space to display data in an understandable manner. Hamster statistics tab could be a good inspiration.
It might be interesting to have an option to display, in the tag view, the total number of tasks for a given tag, including closed one.
eg : 8/10 means 8 opened tasks on a total of 10 tasks for this tag.
It makes especially sense with the option to not display closed tasks after a certain amount of time as the "total number" would reflect of course only the closed tasks still displayed. So meaning : "You have closed 2 tasks of this specific tag in the previous week"