One of the less often discussed, but to me most important features of the N1 is it's awesome call quality. They've got a second mic on the back of the phone to detect ambient noise, as well as some kind of noise suppression tech for both incoming and outgoing audio that's apparently made by by these folks: http://www.audience.com/index.html . They've got a couple demo's on their site of the difference their technology makes.

Disclaimer: I've never tested the call audio quality on any model of iPhone, so I can't make a direct comparison between the two. I can say that my N1's call audio quality is far better than on my old Razor (both on Tmobile).