You could download any application and install it over the air, and each phone would be different and unique to that person. It opened up so many possibilities not only for iPhone, and later iPod touch users, but the wider market.
I wouldn't bet that most people who have bought an iPhone or iPod touch is because of the expandability of the devices with third-party applications, rather than the other features or the aesthetic design of the device.
The iPhone isn't the technology of the decade. It's the vast number of applications which accompany the iPhone and iPod touch, and the mobile applications which followed in the App Store foot steps.
Got a favourite iPhone application? What would rank in your top three technologies of the decade? Comment away.