I think the voicemail integration was the hardest part... And I also would presume that exclusivity is what it took to get AT&T to make the technical changes on their backend. Of course it's also the reason I never got an iPhone. You couldn't pay me to use them as a carrier again, one too many bad experiences by that point.

I got the original G1 phone when it came out on T-Mobile, and it was nice enough (the stupid ball and hump on the side, I didn't like so much), and the physical keypad was nicer imho until gesture input started working well (swype then google's).

Other Android phones started trickling in a little later.