I can't speak for the grandparent poster here but for me, the killer difference between the N1 and the iPhone is that the browser is far, far faster in the N1.
The rendering speed of the N1 amazed me the first time I pulled up a page. It also has enough RAM to keep more than one page open at a time without having to re-load as you switch windows (as I discovered with the iPhone). This may have been addressed somewhat in the 3GS but I only had a 3G model.
I also appreciate OTA syncing for contacts and calendars that doesn't cost me $100/year.
The integration with Google Voice is also a plus (having it auto-switch your voicemail over, and make all outgoing calls through your GV number).
The speed issue is mostly a shortcoming of the 3G. The 3G is so slow under iPhone OS 3.0, I'd have a hard time recommending it to anyone. The 3GS doesn't have the same problem, but as I don't have a N1 it's hard for me to compare them.
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).
Speed mostly, but also the camera, ability to customize, widgets, the screen and hardware, the android 2.1 operating system, the many Google apps such as Google Voice and Google Goggles. I mean there are clearly some UI advantages that the iphone has where the equivalent in android is a little frustrating, but overall I think this Nexus One is a million times better than the 3G. I'm not talking about the iPhone 3GS, which I never owned. I'm comparing the Nexus One to my iPhone 3G.
