All three modern Abrahamic religions have described God in exclusively masculine terms throughout their histories. In She Who Is, Johnson wishes to develop a feminist language about God from within the Roman Catholic theological tradition. Whether or not you practice Catholicism or any other monotheistic religion, you will find Johnson's ideas an excellent lens by which to examine and discuss modern feminism.
In her book, Johnson wishes to "test the capacity of female images to bear and disclose what Christian truth testifies to as the blessed action of God in the world and call upon the language of classical theology to give these images density" (page 13). I commend Johnson's dedication to using the works of classical thinkers to give her arguments legitimacy in the eyes of traditional Catholics, but I fear that her "female images" would fail an objective test of their "capacity."
Here's the deal: believers of all stripes have historically described God with masculine imagery because God has a masculine relationship with humanity. This is not to say God is "male." God transcends all physical and biological categories, so any description of him as male or female is a gross mischaracterization. However, male anthropomorphisations of God are perhaps the least inaccurate ways we can describe Him given our limited capacity for knowledge and understanding.
The most evident differences between the biological male and female show themselves in the reproductive process. In humans, the man initiates the process by proposing his intentions to the woman. The woman then either accepts or rejects his advances. The man then supplies the sperm which activates the egg cell and then causes it to begin dividing, while the female then nourishes and sustains this new life, first within her own body, and then by nursing with substances from within her body.
In all the Abrahamic religions, God reveals himself to humanity, thereby initiating the relationship. God creates, but not out of his own substance as a mother does with a child. God gives humanity power over the creation that he supplies, just as the husband in a "patriarchal" family provides the income and the wife does the shopping, cooking, sewing, etc. The Abrahamic religions teach that all creation is good, so feminine things and concepts are good. Since all goodness comes from God, femininity is from God and contained within God, so God does have a feminine side. Regarding the things that differentiate Him from us, though, He is masculine. That is not to praise the male above the female in humanity, since all human men are as much feminine in relation to God as women are.
Also, within the Christian tradition, the most important and respected human being of all time (who was not also God) was a woman, Mary. Since all humans are feminine in relation to God, they must look to a woman as their example of how to respond to Him. Men, in dealing with God, must go against their nature and suppress their urges to dominate and control. This places the female at an advantage when it comes to piety, a noteworthy characteristic that too many modern feminists disregard when they try to turn women into uterus-bearing men.