From non gamers? Saying the really degrading stuff, like, speaking of toys and looking at you as childish. But then I regard such a person as close-minded and go on.
Among gamers? Destructive critique. It's terrible. As DM I'm constantly concerned with every aspect of the game that could be sygnificant for my players enjoyment. I'm seeking critigue for crying out loud and am frustrated if I don't get it, even beginning to think stuff like "It's so bad they don't want to tell me!". But then, if your critizised without reason or a really bad reason, it's terrible.It's crushing. If you don't enjoy my games, think about it, seek the reason, then tell me. If you can't find the reason, try to figure it out with me. If you don't like the style of my campaign (to many fights, rules etc.), that's fine, tastes differ. But destructive critizism is nothing different from saying "you suck".