God is Love
The apostle John tells us in I John 4:8 that "God is love." This is axiomatic for Christianity; love is fundamental to the nature of God. Trinitarians, though, claim that they have a superior model for explaining God's love than do non-Trinitarians. According to St. Augustine, the Father loves the Son, the Son receives the love and returns it, and the Holy Spirit is the love shared between them. In a unitarian model, God is a single person, all alone in eternity past before anything else was brought into existence. So the Trinitarian argues that the unitarian God's nature can't be love because there was nothing for him to love before he proceeded to create. In other words, the argument is that if God is love, he had to eternally be love, and that's only possible if God had something eternally to love. In the Trinitarian's argument, that would be the differentiation of persons within the Godhead.
Let's hear some unitarian rebuttals to this line of reasoning.