I have appreciated how they cover such a wide variety of topics in a very sensible way. For example, they note how many people today do not know how to do have good, solid friendships, which is a detriment to marriages. Not only does this make it more difficult to have a good relationship with one's partner, it also means that the partner is seen as the only one able to meet one's need for any kind of healthy connection, which places an overwhelming burden on a marriage. Furthermore, it causes a lot of loneliness for those single (Wesley Hill's book, Spiritual Friendship (Brazos Press, 2015) is a wonderful book for those wanting to explore how Christians ought to invest more in friendships).
I especially learned from their thoughts on headship in marriage. I have grown up in fairly conservative circles and have been taught that the man was the head of the spiritual household (as per Ephesians 5). Although the idea of headship has been interpreted in many positive ways, where the sacrificial nature of Christ's love for the church is emphasized and it is clear that the man is listening well to the woman and making a decision that includes her wisdom, I still feel somewhat uncomfortable about the concept of headship. The two quotes below from the Petersons help me understand a bit better why Christian teaching about headship can be problematic:
Mutuality "takes a willingness to talk with one another and listen to one another, for long enough that it can become clear what the issues are, what the feelings and desire of both spouses are, and what some possible plans of action might be. Headship as decision making, by contrast, can seem quick and easy and far less personally demanding. Husband and wife don't really even have to work together: he just does his job and decides, she does her job and goes along, and they're done. And that is exactly the problem. They haven't actually dealt with their differences; they've just done an end run around them. They are no more united when they are done than they were when they began." pages 94-95.
There is "one more unpleasant truth about the control-and-acquiescence mode of male-female relationships. Defining male headship as control and female submission as acquiescence is not just misguided; it is dangerous. By idealizing rigidly defined gender roles, assigning power in relationships disproportionately to me, and encouraging both men and women to see this as spiritually appropriate and desirable, a theological ideology for abuse in intimate relationships is set in place." page 95.The Petersons have identified for me aspects of headship that make me realize why I ought to be uncomfortable with it. Headship gives an excuse to avoid actual communication and decision making together and thus avoid how working together can and should bring people closer together. Secondly, headship tends to move the focus to being about power, instead of on what marriage should be more focused on: mutual submission (Ephesians 5:21), respecting each other, loving each other, and nourishing and caring for the other.