To me, I was disappointed more by iOS4 than the iPhone 4 hardware. The new iPhone's got a beefier chip, twice as much ram, great screen, much better camera, HD video, gyro (who saw that coming?) so there's not much for me to complain about there. I would have liked to see 64gb, though, even if it was $399 since my hypothetical iPhone would also be my main media device. I'm guessing they ran out of physical space for more flash chips...
As for iOS4, the notification system was the biggest disappointment for me. It's very crudely implemented in that it interrupts whatever you're doing separately for each notification (especially if you compare it to Android's implementation). Maybe it's one of those Apple "we weren't first, but we're doing it the best" strategies.
I also would have liked to have seen widgets on the home screen (at lease the lock screen) especially since you can nest all your apps into folders and move them off the primary home screen.
Also it's frustrating that tethering and turn by turn navigation are costly add-on's for the iPhone when they are free on Android. Also, no VZ, but I wasn't really expecting that one...
All this being said, I would prefer the iPhone to be missing a feature here or there rather than have it lose its place as the elegant, straightforward device that it is. Still, Apple has nailed all the big hardware and software weaknesses with this update; I've never owned anything more than a flip phone and I think I might even leave Verizon for this one.