In my opinion, one of the worst mistakes a man makes in grooming is not keeping his nails cut short. Long nails on the toes or the hands is very unattractive. Even if the nails are clean, long nails suggest the man is not working with his hands regularly.
Hands are important because it is front and center in every interaction. When a man holds a woman's hand, passes her the salt at the table, or gives a massage, the woman is thinking about his hands.