I'm sorry to be a negative ninny but from where I'm standing no iPhone model has had a 'Killer feature' for a long time.

Nobody I know is bowled over by retina on such a small screen when compared to similar Samsung or LG screens.
Nobody I know uses Siri for anything other than showing off at the pub.
Angry Birds and other 'killer' apps that made the iPhone more desirable made the port over to other platforms yonks ago.

For me, the iPhone has always been behind the curve on the tech but way ahead of it on the marketing and aesthetics. I realise that some of you who may rely of some of the iPhones 'killer' features may think otherwise and that's OK - this is only my experience.
Between the fact that people see this as the 'premium' phone and look to it as a sort of status symbol and the fact that Apple have now built up a strong, loyal, consumer fanbase any model they bring out is still going to sell well.

That, my friends, is why people keep buying it - the perceived value rather than the actual capability. The brand is the stronger component here. The tech was always the weaker part.