I have two zorder-related complaints:
(1) The default zorder is not reasonable. If I plot a bar chart and then overlay a scatter diagram, the scatter diagram symbols are behind the bars. If I reverse the order, i.e., I generate the scatter diagram first, the scatter diagram symbols are still behind the bars. This doesn't make sense. Something that gets plotted later should be on top of whatever was plotted earlier.
(2) One really has to poke around to find the documentation for zorder. It would be great if the documentation for something as important as this were easier to find.