What annoys me is when the hotel says that the room has been de-feathered but I discover at bedtime that this isn't true. On my last Marriott stay, the front desk said at check in that it had already been done, but I had to make two more requests.

However, my worst experience with this was an Omni that kidnapped my polyester pillows without warning in the middle of my stay. I didn't notice it until I woke up coughing in the middle of the night, not only with red eyes and being unable to breathe, but my entire face was puffy and swollen.

BTW, in addition to the pillows and duvets, mattress pads can contain feathers.