Yeah i suggested that also here: https://plus.google.com/u/0/110230424911720375919/posts/MUKJ42m1f2X
Suggestion: Subtract circles from broader universes
Negative filters look nice at a glance. But add complexity... that should not be visible to "noob" users. Depth should come thru expert usage:
Shift-Click existing circle to modify it, or something like that (long press in mobile).
When adding circles to share post to, shiftclick one to "turn it red", thus subtract it from bigger universe circle.