Hi Dan,
Have you ever considered putting in a switch to invert the selection of certain filter? As an example, say there are 10 people assigned to. I want to see my tasks, so I filter on myself. I then want to see the tasks that aren't mine. Currently I need to deselect myself, then select all the others. Better would be to be able to invert the selection to not(me).
In terms of an interface, there could be a selectable entry near the top of the list like <invert> or <NOT>, and the filter box could show "Not: XXX, YYY..."
This may not be appropriate for all filters.