That's a great question -- but I'm not sure I can pick one thing.
So I'm going to cheat a bit and share the three things that impressed me the most about the iPhone and its ecosystem.
* First, the degree to which Apple and iPhone users have made the iPhone (and iPad) indispensable to daily life. Ten years ago, having a go-everywhere device that could take high-quality photos, record pro-quality audio, create and edit business documents, and keep you in touch (phone, IM, e-mail) all day long seemed a huge stretch. But now we're demanding an even more powerful version!
* Second, the fantastic range of apps available for iOS -- both on the App Store and on Cydia.
* Third, the amazing hardware we can get for our iPhones. For example, being able to mount your iPhone on a tripod easily is great, but being able to get a case with three switchable lenses built takes photography with the iPhone to a whole new level.
Here's hoping the next iPhone and iOS 7 keep up the progress!