No killer app for iPhone?
The fact is that modern computer user habits are staggeringly fragmented once you move out of the email/browser/office apps arena. Just like browsing habits-- who goes to a generalized portal like AOL.com or Yahoo.com on a regular basis? Computer use is now specialized. A video editor will use apps pertinent to his job, while a cook would likely use a suite of apps for food planning and prep and purchasing. There's not likely to be a killer app outside of your specialization, unless it has to do with massaging data-- something along the lines of syncing your contacts, for example.
Ultimately the new killer apps are fixes for existing applications. One of the reasons 90% of iPhone users browse the web on their phone? Mobile Safari is hands-down the best portable web browser out there. It still isn't as robust as the desktop equivalent (hence the plethora of iPhone-tuned pages out there), but it is much better than the competition. The ultimate killer app is your window to the web, and the competition keeps making things better all the time. Hear that Mozilla? We're ready for mobile Firefox already!