The usage of setOpaque is essentially an implementation detail to speed up drawing but because certain standard components change their drawing behavior when setOpaque is set to false this has created confusion about setOpaque really means. Adding to this confusion is the advent of L&Fs which use OS derived skins. It is unclear how a skinned component should be drawn when a developer sets the background color to something other than the default. Should the color be ignored, should it replace the skin, or should it augment the skin somehow. Currently this behavior is inconsistent and depends on the component and the current look and feel and running platform. To further complicate matters, JButton has extra properties to control when the content should be painted, but only JButton has these properties.  

Once a compatibility API is in place we should define and document exactly how all of these properties fit together, especially in the light of forward looking native look and feels for Mac OS X and Vista.

Once this issue is settled several related bugs regarding buttons and toolbars will have solutions. (see attached bug ids).