For me:
1. The vulva and women's bodies in general were smaller than I had imagined (i.e., I didn't fully appreciate the size difference between a man's and a woman's body)
2. I had never fantasized about rear entry, but the first time a woman presented herself to me to be taken from the rear was like a revelation -- the glistening pink labia, the triangle of dark hair and the smooth buttocks
3. In the woman on top position, the feeling of vaginal fluid trickling down my scrotum was unanticipated and delightful.