What are the similarities between most Christian beliefs and Dianic Wicca?

Very interesting question. (You probably should refer to Dianic Wicca in Wikipedia.)

Looking for similarities between Dianic Wicca and common Christianity, one might consider the close similarity of high Holy Days, with Samhain corresponding to the Feast of All Saints, Winter Solstice with Christmas, Spring Equinox with Easter and so forth. This correspondence is not accidental, because the Christian Liturgical Calendar is based on both Hebrew and pre-Christian religious cycles relating to the annual agricultural, lunar and solar cycles.

Another interesting similarity is the figure of the Madonna and Child. In Wicca, this figure is the Goddess and her Son, with the Goddess being the greater deity. Christian tradition reversed this figure, interpreting it as the Virgin Mary and the Infant Christ, with the Child being the dominant Deity.