I’d love something like Flash’s symbols in OmniGraffle — a way to update, say, 100 instances of an object by editing the master object. Being able to select objects based on properties is close, but only when changing stroke/fill/etc., not, for example, when adding lines to an object that you’ve already manually duplicated 100 times.
Anything like that coming soon? Or is it there under some name I haven’t thought of/couldn’t find?