First I want to say, that all your kids will be fine in the end. There I said it! Unless there is severe mental illness or severe family trauma, by the time your teen hits their 20's they will adore being at home, desirous of your input and advice about their life, and unbelievably fun to hang out with. So take a deep breath, pleeese!
The way I see it, in my experience, after 30 years of working with families, and raising one of my own, there are only three real parenting mistakes that can change the outcome of your child's life.
First is the too strict or rigid parenting style. If you are the party of NO, my way or the highway, or you have a ton of rules way past the time that kids need rules for everything, and you have extremely high expectations for your teen's academic performance, you run some risks.
Risk #1: Adolescence is all about independence. If you continue to write the script for your teen's life they will react in one of two ways. If they feel over-controlled, over-managed, and have to answer to too many rules, some kids will be forced to act out to get the freedom their brain and their body are telling them they should have. By acting out I mean lying, hostility and anger, deliberate school failure, drug and alcohol use or abuse and avoiding you at every turn. This can feel like armed warfare. These teens need to learn how to make decisions on their own. These are the kids that often bail on college. As soon as they hit campus, and experience that first taste of freedom, all control and discipline, no matter how much you have drilled it in to them is gone. They have never actually learned how to be self-disciplined, or internalized the rules and structure that you imposed. As young children structure and control is good, as teens you need to share and encourage with supervision your teen's innate drive to be independent.
Risk #2: Some teens who are over controlled and over-managed become extremely passive. They have developed what is call learned-helplessness. What they integrate is a lack of complete confidence in their ability to make decisions, and look to you for direction in all parts of their life. This is not healthy. These teens are lovely to have in the home because they never fight with you, and come to you often for help. For a parent, there is nothing like it. However in life, you will not always be available. When it comes to adult relationships whether romantically, with friends, or with bosses or colleagues they will rarely speak up for themselves, and open themselves up to be taken advantage of, thinking that they don't know better. These kids need to learn to have confidence in their own ability to make decisions, and that what they want matters.
Risk #3 is the too permissive parent. This parent maybe has an unspoken rule, do well in school, and I will ignore everything else. Or maybe, your philosophy is that your teens should be able to manage their own lives, or maybe the parent's life is in chaos with a divorce, or other family crisis, and takes their "eye of the ball" being too involved in their own life events. Rather than too many rules, there are no rules, no expectations, no supervision. These are the parents with the blind eye. A blind eye to what goes on in their own basements with their teens and their friends, a blind eye to where and what their teens are doing when they are out and about, and a blind eye to their teens safety. Teens are by nature risk-takers. Sometimes those risks can be life-threatening, either physically or psychologically. Teens need to know that someone is looking out for their welfare, even if they fight you tooth and nail when you do. When these kids move into adulthood, they are often entitled, irresponsible young adults, who look towards you to bail them out when they act badly, perhaps its is financially, or legally. These now grown up kids, can't or don't feel like managing the mundane of life, and will constantly look to you to do it for them, even well into adulthood.
So these are the three biggies. Everything else in between, usually works itself out. Parenting a teen is about setting enough limits to keep your teen safe, and give enough leeway for them to practice decision making, knowing that they will make mistakes, that they will hate you some days, and knowing that underlying it all is love. Your love for them, and their love for you. It really is as simple as that.
This is a very interesting article about the consequence of "over-parenting" when your kids hit the real world.