Besides the fact that I'm already locked into a T-Mobile plan that is serving me well (well, it's ok!) there's a few key features that iPhone 3G should have...
expandable memory card slot (like a Micro SD)
video ( in this day and age no mobile video... come on, you gotta have it)
5 mega pixel digital camera is a must (currently comes with a 2 MP)
spotty 3G coverage
costly monthly plan - you'll be dropping $1000 easily this year on your iPhone 3g
Hang tough and I'm sure I'll come up with a few other key features. The real kicker with iPhone 3G though is iPhone 2.0, the software upgrade that enables mobile application developers to create apps for this platform... its the apps man, that's what will be killer! Think about it, with this great UI and nice, large screen you can now easily read the New York Times from cover to cover on a phone.. try doing this on any other device, your eyes will likely bug out of your head by page 3! Check out LBSzone.com for details of just a few, new, cool LBS apps running on iPhone 3g. Any thoughts???