First Look: iOS4 on iPhone 3G vs 3GS
The biggest let down is that multitasking isn't available on the iPhone 3G. Apple says its older processor just can't handle the feature. With iOS4 on the iPhone 3GS and iPhone 4, double-tapping the home button brings up a list of open apps that users can jump to quickly. This simply isn't present on the iPhone 3G. The multitasking is probably more accurately described as faster switching between application. Only certain applications can truly run in the background, and the app switching doesn't necessarily show you other open apps so much as it lets you jump to recently used apps faster. Probably the most important aspect of this is that the iPhone preserves the "state" in which you leave apps. In other words, if you've started typing an email, and jump to the browser, you won't lose your spot in the email.
For some reason, the new home screen wallpaper tool isn't available to the iPhone 3G. Not the biggest let-down in the world, but it's a seemingly odd omission for what appears to be a simple feature.
The iPhone 3G does not support Bluetooth keyboards, which the iPhone 3GS, iPad, and (presumably) iPhone 4 do. Again, not the biggest disappointment. Apple hasn't explained why this feature isn't supported, but it is possible it has to do with the components inside the iPhone 3G. With the iPhone 3GS, users can pair any Bluetooth keyboard and use that for text entry. What's great is a lot of keyboard commands, such as Control+C for Copy and Control+V for paste are supported.
With iOS on the iPhone 3GS, the screen can be locked in either portrait or landscape orientation. This feature is built into the multitasking tool in iOS4. It is not available to the iPhone 3G. This means that the screen will continue to automatically rotate when the phone is rotated.
Thankfully, the list of features the iPhone 3G gains with iOS4 is longer than the list of features it doesn't gain.