We propose a simple physical model that predicts the optical properties of a class of microstructured waveguides consisting of high-index inclusions that surround a low-index core. On the basis of this model, it is found that a large regime exists where transmission minima are determined by the geometry of the individual high-index inclusions. The locations of these minima are found to be largely unaffected by the relative position of the inclusions. As a result of this insight the difficult problem of analyzing the properties of complex structures can be reduced to the much simpler problem of analyzing the properties of an individual high-index inclusion in the structure.