Human disutility includes more than just pain too. Destruction of the humanity (the flat plain you describe) carries a great deal of negative utility for me, even if I disappear without feeling any pain at all. There's more disutility if all life is destroyed, and more if the universe as a whole is destroyed... I don't think there's any fundamental asymmetry. Pain and pleasure are the most immediate ways of affecting value, and probably the ones that can be achieved most efficiently in computronium, so external states probably don't come into play much at all if you take a purely utilitarian view.