Weird mailing prohibitions in foreign countries (2009) (newfoundlandnews.blogspot.com)

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