The overwealming majority answered that it was the Christian affirmation of the divinity of Jesus that they found most difficult, with many expressing their objections to trinitarian and incarnational understandings of God.
Some comments suggested elements of misunderstanding, even bewilderment, especially when it came to the crucifixion and what God was supposed to have achieved through it. But even where that was the case I doubt better understanding would have bridged the fundamental gap. It was clear that the gospel itself, the good news of God among us, was the issue. Our primary teaching is the primary offense.
Even so, the responses were respectful and a number expressed appreciation for the question, one even suggesting a reciprocation in kind. Most surprising to me, many expressed positive experiences with Christians, as Muslims, again making it clear to me that it was the good news of God among us which was the most significant stumbling block.
How do we overcome this core difficultly that Muslims have with Christianity? Clearly not by education alone, for the primary problem is not ignorance. Clearly not by hospitality alone, for the primary problem is not hostility either. Clearly something more is required. Personally I suspect nothing less than a new experience of God, a sign that surprises.