That's a subjective question, so I'll give a subjective answer. The browser, for better or worse, was a lot less dumbed down for mobile than competitors, the stylus-less touch interface reduced UI friction and the odds that you'd lose a critical (if inexpensive) component, and the slew of contemporary iPod users could easily migrate their libraries over.
Perfected multi-touch touchscreen
Before that we had touchscreen but they sucked.
---
I really don't think multitouch is what made or broke the original iPhone.
That's a subjective question, so I'll give a subjective answer. The browser, for better or worse, was a lot less dumbed down for mobile than competitors, the stylus-less touch interface reduced UI friction and the odds that you'd lose a critical (if inexpensive) component, and the slew of contemporary iPod users could easily migrate their libraries over.