We probably have a different view on what is a killer app. From my perspective: Windows - Outlook Express, IE. OS X - Mail, Safari. I can't recall the last time I fired up "Photoshop, Excel, Quicken, Final Cut" and I bet 90% of randomly selected users couldn't either.
"Killer app" usually means the app that motivates you to switch platforms. For PS3 or Xbox 360, it's the game that made you want to buy the console. For Windows in the '90s, you needed MS Office to get your work done, so you bought a PC. All other things being equal -- i.e. assuming the system has a web browser and the other essential software has cross-platform equivalents -- the killer app is what kills off rival platforms.
When I hear someone say "I wish I had an iPhone right now", it's usually because they're either (i) lost, or looking for something, and want access to Google Maps, or (ii) in the throes of gadget envy. The App Store addresses (ii), but not with any single app -- it's just the idea that there are thousands of fun toys that only iPhone users get to play with.
So there are a couple of killer features, but nothing that can't be replicated on other phones. Not necessarily as well, but that's not the point: without exclusivity, the iPhone will not get the same kind of dominance Windows has.
As reliance on mobile devices and their capabilities increase, this could become less of an issue.
It is still a luxury for most people - "oh cool look at this app," but it still comes down to those that can afford an iPhone. I can imagine there are many family squabbles involving parents that bought their kid an iPhone that racked up an App Store bill the parents didn't originally intend.
iPhone is not a business phone overall - and therefore doesn't have "critical apps".
The point I was replying to was that "... third party apps are just as often or more used on iPhone than on, say, Windows and Mac OS X".
I really doubt this is true given the lack of any killer mobile apps. I don't personally consider any of desktop apps in the list I gave 'killer', but fairly sure they've sold lots of computers.
