The proximity sensor was the key that made the iPhone work. When people say some other company invented the touchscreen smart phone first, I like to ask, did it have a proximity sensor?
In reality, it's the combination of all those things and more that made the iPhone work, made it more appealing than anything Palm, RIM or Danger made. Just adding a proximity sensor, or one of the other things to that list, to a Palm Treo wouldn't have made it an iPhone.