What, if any, are the rules on ATC politeness?
I worked as a controller in the U.S. for many years in the 1970's and 1980's at 3 different busy facilities (twr/artcc/tracon).  There was never a written policy regarding politeness on the radio.  It was, at least for me, common sense to not engage in any extraneous conversation.  Controllers are keenly aware everything that is said on the radio and landlines is being taped.

The few times I was aware that a pilot made a complaint regarding a particular controller, the controller's supervisor would get the tapes, listen to the pertinent section, and then if warranted counsel the controller.

Generally, it was my experience, that when working traffic in a busy facility you would just want to get the job done as professionally as possible and avoid any extra conversation.