Lack of Discipline
Discipline is about developing self-control and learning prosocial behavior, not about rules or punishment. It's about taking on responsibility, learning to mix work with pleasure, and eating your vegetables when you'd rather have a doughnut. Rules and punishment are only intended to be a means to a disciplined child. Our desire with discipline is a goal much richer and much more important than merely being able to follow the rules. It's about raising a child who is kind to others and self-disciplined enough to be successful in life. This makes discipline an extremely important aspect of a child's upbringing, and neglect in this area is one of the most overlooked methods of lasting injury to a child.
There are two types of discipline children need. The first is responsibility-based discipline: brushing their teeth, doing chores, looking after the pet, completing homework, etc. The second is behavioral-based discipline, designed to develop prosocial behavior in children: learning to be kind, no hitting, no stealing, treating others with respect, etc. Both types of discipline, and a child's level of discipline in these areas, will affect every aspect of their adult life, from their ability to hold down a job and stay out of legal trouble to their ability to make friends and be successful in relationships.