One of the features of the Mac OS GUI that really stands out (sorry for the pun) is drop-shadows for menus. They help to give a more 3D effect. Not having a Mac myself, I do not know how successfully they work on heterogeneous backgrounds. Theoretically an object's shadow should depend on the relative height between the object and the local background. That leads to complications in processing that maybe outweigh the advantages of the effect. Perhaps having a semitransparent shadow of constant width is an acceptable compromise.