Success of iPhone is phenomenal and beyond the imagination of consumers, analysts and competition alike. While 'Steve Jobs' and his pursuit for perfection has been the driving force, I'd attribute three reasons to its continued success - Design, AppStore and Marketing.
Design: 'Keep it Simple, Stupid'. There are more powerful mobile phones all over the world that are incredibly more powerful and feature rich than iPhone. But smart phone software had mostly been stripped down versions of the desktop software, ‘User Interaction’ paradigms from the personal computers with windows and menus with a key board are translated to mobile phones. iPhone redefined the mobile user interaction and kept it simple.
App Store: 1st generation iPhone is incredibly successful even with limited features. To sustain the innovation and maintain its leadership in a market that has innovators like Google, Microsoft, Nokia, Sony and RIM is a challenge. Opening up the platform and letting hundreds of other companies innovate is only part of the story. Providing a platform for these applications in iTunes AppStore, making it incredibly simple to explore, buy and install apps replicating the iPod/iTunes eco system is critical for the explosion of the app market. This increases the iPhone’s capacity to expand and retain the user base .
Marketing: Smart phones before iPhone have positioned themselves for business users and have been competing to capture the small but profitable market. iPhone instead marketed its smart phone for mass market. Its approach to market these ‘smart phone’ features to the mass market has opened up the incredibly large market. Apple's continued reliance on the ‘Apps’ instead of ‘coolness’ in its marketing campaign has taken iPhone beyond traditional Mac/iPod consumers would ensure its long term success.