Link to the current list of restrictions: http://pe.usps.com/cpim/ftp/manuals/imm/full/imm.pdf
A few actual quotes that I've found in just 10 minutes of browsing:
San Marino: "Toys not made wholly of wood"
Qatar: "Musical greeting cards"
Albania: "Extravagant clothes and other articles contrary to Albanians' taste" (already in the article but I thought the literal quote was even better)
The list is a gold mine, it seems every 5th country has some strange restriction