Definitely the user interface (iOS). There wasn’t anything the original iPhone could do that competitors couldn’t, including multitouch.
I think the real revolution was the user interface. At the time the competition was using fairly utilitarian, bland, and uninspired design. The iPhone dramatically changed that. It demonstrated that you could have function with form. So while the iPhone launched into a smartphone space that was both emerging in terms of smartphone adoption, it was also maturing as competitors like BlackBerry and Nokia had several years of smartphone development experience under their belts and with sales figures to prove it.
The simplicity of iOS and the launch of the iPhone from my perspective opened a door that smartphone users had not seen before. It was a proof of concept of simplicity and functionality. Making the smartphone, a tool for communication, looked at as something more accessible.