iOS 4.2 - What's there for the iPhone 3GS?
Among other things:


Free “Find my iPhone” (don’t need MobileMe) —> this requires an iPhone 4
Safari allows you to search inside the page
You can assign different SMS/MMS/ALERTS to different phonebook contacts
AirPlay / AirPrint?
A volume slider control in the Multitaskbar
New icon for Voice Memos
 - 


Anything else?  

UPDATE: There’s a trick to use the Find my iPhone in a 3GS or similar. I found out that if you create everything from an iPhone 4 (and/or presumably an iPod TOuch latest gen), you can later go to your 3Gs and enter the same data and it will work. :)