Sonnet 20 is the source of much debate. It describes a man that has been gifted with womanly features - a delicate looking man. This man has earned the author's love, though the author asserts in the last lines that it is a spiritual and not a physical love (But since she prick'd thee out for women's pleasure/ Mine be thy love and thy love's use their treasure.)
In the opening lines, the author identifies women as the both the weaker and the less honorable sex. He insists that the man being referred to is not tainted by "false women's fashion". He suggests that men are more honest and more intelligent ("eyes more bright").
Power is refernced in this man's power over the people around him, catching the attention of men and women alike. It suggests that a person's power in society is tied in with their appearance, and can even suggest that a person bearing both manly and womanly attributes is the most powerful, able to influence both sexes.
As for identity - how much is our identity tied into our gender? This is a "master-mistress". Can such a man-woman exist? If so, how is that person to behave in society? Shakespeare is drawing our attention to the limitations that are inherent in our sex, and the expectations. This master-mistress is meant for women, as dictated by Nature's "application" of the male genital organs; however, that doesn't mean he can't be loved by another man in a spiritual way.
Some believe that this sonnet is proof of Shakespeare's homosexuality; others believe Shakespeare was just trying to elucidate the universal nature of love.