Water usage is probably my biggest. Living in a high desert, my wife and MIL see no problem with filling one side of the sink with hot soapy water to wash a few dishes because “that’s just how I’ve always done it”, to watering the grass and plants for hours. All of this makes me mental.
Water usage but in the opposite direction. Yes, I’m going to take a bath. No, I don’t think it makes any actual difference ; go blow up a factory or a data centre if you care so much about water. (Or maybe do it even if you don’t, it never hurts !)