A similar situation happened one time when I was trying to study with a Mormon. He wanted to talk about hundreds of issues, but I continued to say that unless we resolved whether or not the Book of Mormon held the authority of Sacred Scripture, then we would have a really hard time discussing things.
On the flip side of that, sometimes we should start discussions by talking about all the similarities between the two groups. One way I like to study with Protestants is by reciting the Nicene Creed so that we have a huge bounty of issues that we can agree on, then try to get to the question of why we believe it. All in all, however, I really believe that the binding authority of scripture, the Church, or other sources has to be worked out in order to have a real fruitful discussion.