Communication. 😉
Given, no? But to elaborate, it makes texting and email so seamless and easy. I almost never use the iPod function. And I do use Safari at times, but really, I've found the texting and in particular, mail app to be so convenient.
(And seamless integration w/ Mac apps like address book, iCal, etc.-- apps I never used until the iPhone pretty much shoved it in my face. 😀)