That definition assumes that there is such a thing as an "app worth buying a phone for" that holds for a majority of people. That will always vary by person. I've listed killer apps for Android for me; as I said, I can make no claims about killer apps for everyone, nor can you, or for that matter, Gruber. So I guess the term is kind of silly in the first place when applied to third-party mobile app offerings, no?
The iPhone had a killer app in the full web(minus flash) web browser. No other phone at the time had a web browser anywhere near as good as mobile safari is. That and google maps and mail made a compelling case to buy the phone.
Was it multitouch screen? The integration of multitouch + the web was pretty amazing and I would consider it a killer app over the arrow based web browsing that came before the iPhone. Full web + multitouch(zooming and selection of links are amazing) + high DPI screen + large enough to get things done without being too big to fit in a pocket == killer app.
When I say that there wasn't a competitor that was close I mean in total experience not just having the web. In my opinion all the little things matter immensely.
