There are no features that were previously available in the iPhone 3G that are no longer available. Ther are features in iOS 4, such as wallpapers and multitasking, that the iPhone 3G will not support, not to a level that Apple considers acceptable. Yes, some features can be had by jailbreaking, but they do not work well; people just live with the problems.
If you were me, you couldn't make an update, by the way, since you would not work for Apple (no one who responds here does, as I said) and could not do anything of that sort.
Anyway, you've been given the way to comment to Apple. There's nothing more anyone here can say.
Regards.