One idea I haven’t seen discussed on GTDNext, but that I’ve found really useful with other GTD tools, is a chronological listing of completed tasks. I find this invaluable for keeping my daily log, doing weekly reports, etc.
The Archive section is quite useful, but currently has a different purpose. I’ve no idea if, for example, the Archive could get a “view” added that shows a chronological listing of completed tasks.
In any case, I hope you will consider adding this feature and that it won’t be too difficult to do so.