"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Apple Looking Into Reports of Problems With iOS 4 on iPhone 3G
The most common criticism is that the phone is slow after an upgrade. There are also many complaints that the phone drains the battery quickly and becomes excessively hot. Concern about general problems such as slowness and battery life on gadgets isn't unusual, but using iOS 4 on the iPhone 3G seems to make the problems so bad that the phone is nearly unusable for some people.In venting their frustration, some users have even gone so far as to create parody videos demonstrating the iPhone 3G's inability to run iOS 4 smoothly.
Meanwhile, some reports have indicated that iOS 4.1, currently in beta testing by developers, will address these issues to some degree.
Apple has made clear since the initial introduction of iOS 4 in April that the new operating system is not fully compatible with all generations of the iPhone and iPod touch due to hardware limitations, with only limited compatibility for the iPhone 3G and second-generation iPod touch suggesting that those models are being pushed to their limits with some of iOS 4's features. Users, however, understandably expected that Apple's steps to disable certain resource-intensive features on those devices should have left the remaining features operating relatively smoothly. That does not, however, appear to be the case for some users.