For me:
Incredible build quality (yes the front panel is gorilla glass
http://www.tested.com/news/whats-so-special-about-iphone-4s-aluminosilicate-glass/429/ )
Extremely polished and consistent user experience.
The phone is equipped to handle my professional and personal needs flawlessly. (great email client, calendar, etc. Also iPod and games for when I want to cut loose).
I'm actually glad that the iPhone isn'd bogged down with a million customization options. I don't need different alerts for texts from different contacts etc. I just need the phone to work consistently, which the iPhone has done. My Android, not so much.
I'm a Mac user as well, so this phone integrates seamlessly with my entire electronic life.
It may not have the fastest processor, or any number of other specs, but what it does have is the best smartphone experience available.