"To be honest, as a user, iOS 4 adds nothing that truly stands out as "THIS is why I must have the iPhone" except for Facetime and the Retina Display."

I am confused. You started your post as a developer and end being an user.

There are some big things in iOS4. Multitasking (done properly), iAds, Game Center and 1500 New APIs and tonnes of improvements that apple has put in after taking experiences of millions apps created by thousands of developers. And That's HUGE.