In my experience the biggest hurdle to constructive discussion of abortion is disagreement over what does or does not constitute human life, or the point at which a multiplying group of cells is considered a human. While there are some extenuating circumstances to discuss beyond that, a great deal of the arguing is irrelevant when one person views abortion as a form of murder and the other person does not believe the life being terminated attains the level of personhood necessary for "murder" to be an appropriate label.
Granted, in the quickness to jump to the M-word, I see few pro-life advocates recognize that even where recognized murder is concerned, our laws have nuance (1st degree is not 3rd degree is not manslaughter; self-defense/threat is factored in; etc.). But often the argument never gets that far, because of that disagreement over how to label the fetus.