I think that a shortcoming of much of the literature espousing a "systems" perspective is weakened by one point:  There is a persistent failure to acknowledge that people in general realize that, for lack of a better phrase, "everything affects everything else."   

The importance of systems theory does not derive from this realization, but rather from founding a way of thinking based on systems rather than discrete entities.  This subtle difference was, for me, the biggest impediment towards appreciating this perspective, which for me is epitomized by Gregory Bateson.