Mrs. Hardcastle's main complaint against her husband is that he almost never takes the family into town to "rub off the rust a little." Mrs. Hardcastle is a fashionable and sociable woman, obsessed with these qualities to the point of greed and vanity. She dislikes the plain mansion they live in—even claiming it resembles an inn in a moment of foreshadowing—and thinks the company they receive is uninspiring compared to the people they could meet with in the city.
By contrast, Mr. Hardcastle is a proudly plain man with an aversion to fuss and fashion. He prefers both the countryside and all things old-fashioned, to the point where he only lets his daughter, Kate, wear fashionable modern clothes during the day if she agrees to put on more traditional fare at night. This difference in preference creates conflict between the husband and wife.
Mrs. Hardcastle's grievance connects to the play's theme of country life versus city life. For people like Mrs. Hardcastle, the city is associated with fashion and glamor while the countryside is associated with plainness and dreariness. However, the author subverts this idea by presenting the country-bred characters like Tony and Kate as far cleverer and more resourceful than the city dwellers.