Surely, it can't be features alone, as many phones have far more impressive feature sets on paper.
Yes, it is the features that make the iPhone what it is and no other phone has come close to matching the features of the iPhone.
Touch: Other phones have so called touch screens but when compared they are "push" screens. Only iPhone has a true touch screen.
Browser: Other phones have browsers buth when compared to the iPhone there is just no comparison. Not sure how long this will last.
UI: No other phone is as easy to use.
Hardware: How many other phones are as thin and also have a 3.5" screen.
iPod: This is pretty much impossible for others to compete with or you would have competitors to the standalone iPod.
App Store: 20,000 and counting and ease of use. I know others are just now trying to get into the game.
It is going to very difficult for any phone to be a true iPhone Killer unless Apple decides to never change it again and the App store drys up. However, I would love to see some true competition to keep Apple from getting lazy.