Land. They regard land as inviolate once they claim it. Now any rational being would understand that not one stone in Jerusalem will be changed as a result of simply designating it as Israel’s capital. Nothing will change wrt borders, etc;
I once had lunch with a Palestinian fellow, about my age. We were at a realtor meeting and ate at a hamburger joint. (What was funny was that the cash register receipt had the clerk’s name which was Jesus (Hay-Soos in espanol) and I joked that Jesus was selling us hamburgers.)
While I did not want to get into argumentation with him over all the standard irritations and gripes because we were getting along fine and it was a quasi-professional type of meeting we (and many others) were at, he *did* happen to mention that Jerusalem was a top-level deal for the Palis. Top-top. I didn’t understand it then and I don’t particularly pretend to understand it now, but I’m just saying that he mentioned it and I could not get any explanation other then “it just is”. Ultimately, I think it is their “mini-Mecca”. It is not so much that for other Arabs, but for the Palis, it is among the biggest of big deals.
Muslims consider it the third holiest site after Mecca and Medina. We can argue all day over the validity of their beliefs, but they do claim it. This isn't going to just blow over like Kushner seems to think.