I added the "Hide windows and show desktop" action to my panel but then it never showed up. Then I noticed I already had it and some other actions too. They were not displayed. But now I see I can hover over them and see tool tips. The icons are not installed. Note that the xfce-system-exit icon is the only one displayed (and the xterm icon for the xterms I have open). Maybe the xfce4-panel should include some simple icons at least for its defaults? Or it should print to stderr about missing icons? Maybe I have a misconfiguration, but it would be nice if the xfce4-panel reported that. If it was a build problem, then maybe it should have failed instead of installing.
Does this look like the 4.4 migration problems in bug 5067?
Improved icon fallback for the show desktop plugin in commit 29739.
Closing my bug. I am guessing this was caused from upgrading from older Xfce. I clicked on properties on the icons and the configuration showed "No icon" for these. So clicked on that I chose a new icon and all is well. I am guessing now that the default icons I had before somehow got lost. All is okay for me now.