I'll provide four fundamental points and a vision.
1) Anyone trapped in addiction or prone to violence is putting his or her children at risk. Parenthood is not just about "parenting practices." It is first about living as whole, independent adults, emotionally and practically.
2) Parents do well to connect first, then direct. The most common unintelligent parenting practice I see is a parent calling out to, or yelling to, a child to do something or other. Children are not aware in the way adults are. Such yelling is just noise, outside their system. When I want a child's attention, I ask for it. I kneel down to their level. Except in an emergency, I listen first. Once I have the child's attention, I direct, if direction is needed.
3) The best parents are free of judgment, blame, and fear. GMWilliams, this is a challenge to you. I appreciate your question and your concern for children. But your language sounds judgmental and hopeless. I encourage you to be more encouraging.
4) Carl Jung said that the greatest problem of any child is the unfulfilled dreams of his or her parents. Parents who press their own agendas on their children is another broad category in direct answer to your question. So parents should know their dreams and be making them real, and then lead children by example to rich, fulfilling lives.
GMWilliams, I challenge your notion of "irreparable harm." Wilma Rudolph left polio behind to become an Olympic runner. Many children who survived the holocaust became healthy adults. Never underestimate the power of the human spirit to heal.
But we want to prevent all the damage being done to our children, if we can. In that light, my wife and I once had a humorous conversation about a training program that would really prepare parents for parents.
After extensive counseling & support ensure that the couple will be romantically happy, stay together, be financially successful, & lead fulfilling lives, the parenting program would begin.
Enlightenment of both parents is the 1st requirement. This prevents projection.
After that, gardening. We want to make sure these folks are tender and nurturing, and willing to learn how things grow, and willing to work hard, before bringing animals into their care.
After that, raising a kitten into a cat. Learn to care for a loved one who will ignore you! Raise a large dog with excellent obedience. Raise a chimpanzee, so much like a toddler in attitude, and a teenager in strength.
At last, ready for a child!