iPhone's killer features are it's first party app's and not the third party app's.
Most competitors forget that iPhone comes with some very special app's out of the box. App's like Calendar, iPod, Browser, SMS, Stock, Weather, Maps, Youtube etc. Most would agree that they don't have all the features. But you really would be hard pressed to find any app's that do these things better. I know apple doesn't allow some of the app's that compete with their first party app's. But it really would be hard to better those app's. If you do a survey of the people using iPhone you would definitely find the first party app's leading the way.
These first party app's are so powerful that, you could do most of things all the app's in the app store would do. Other than the gaming part everything else can be done on iPhone without any app's.
So the real success of the iPhone is it's app's out of the box. It's really surprising why they have not added any new app's. Video recording (cycoder jail broken app has proven that it can be done as beautifully as the camera app) Todo list, Photoshop like program etc, would be good additions. But what they provide is spectacular when you compare it with it's competitors.
For example G1 (Google phone) doesn't even come with a music player and you expect that to compete with iPhone.
The same thing can be said of other competitors like Nokia, Microsoft, Htc etc.
If apple keeps adding to the first party app's on feature updates, it will be impossible for the competition to even compete.