I’ll bite. I am always shocked at the over-entitled people who hog the overhead space with three carry on bags (common in the USA, on UA at least) and who affect to be inconvenienced when I get something or other out of my very small bag on a 11 hour flight.
Also I hate the people who want me to close the window shade on a day flight so that they can watch some mindless drivel on the IFE (often adults watching comics).