I believe that the hotspot feature is only for the iPhone 4. As far as I can tell, that is the only feature of the new OS that you won't be able to use. Hopefully someone will correct me if I'm wrong. Others are reporting that Safari is much quicker with the update. While the chart below doesn't have a 3Gs on it, I think that you would see an improvement as well.
the Personal Hotspot feature does work on iPhone 3GS with iOS 4.3, however ONLY over Bluetooth and USB (not WiFi) - basically like tethering in iOS 4.2 and earlier, there is no separate setting now for tethering.
however if you also update your iPad to iOS 4.3 you can now Bluetooth tether it to your iPhone 3GS. so Personal Hotspot is not strictly limited to iPhone 4 - it's just more limited with the 3GS.
iPhone 3GS also gets faster javascript performance. I ran this sunspider benchmark before updating, and found 2.26x increase in the score.
Personal Hotspot is a term for WiFi tethering. Thus, the 3Gs does not have a Personal Hotspot feature. Only the same old tethering that it's had since day one. All Apple did was change the text.