I don't ever complain about what is served to me in a restaurant. On the very rare occasion I am not happy with the food, I simply make a mental note never to return to that establishment again. I am usually so grateful that I don't have to cook the food or wash the dishes, that a sub par meal is seen as just one of those things to chalk up to experience.
DD (20) says that all of her friends parents complain if they are not happy with the food in a restaurant. They often get a discount or a free meal. I just haven't got it in me to do this.