I am actually quite dissapointed with the IOS 4.0 on my iPhone 3G. After one long hour of upgrading, my iPhone 3G have a lot of problem.
These are my top 5 reasons why I should downgrade back to 3.13:
1. It is now quite sluggish and slow.
Some games and apps are quite slow to load. It also jerk.
2. Phone receptions become worsen.
Not sure whether it is phone provider issue or due to my upgrade, but the problem did come since I upgrade my phone.
3. No multitasking features for iPhone 3G.
Due to the limited memory of iPhone 3G (128MB), apple decided to disable multitasking feature for iPhone3G. Haizzz.
4. No wall paper for iPhone 3G on ios4
This is one of main reason why people upgrades to IOS4. But this is not a new features anyway, a jailbroken iPhone 3G on previous version of IOS can already have a wall paper background.
5. No bluetooth support
Why is that so ? Deactivated support for iOS 4′s Bluetooth keyboard feature is also a hardware issue. The first two generations of iPhones supplied basic Bluetooth 2.0 support. Starting with the iPhone 3GS, Apple added Bluetooth 2.1 + EDR (Enhanced Data Rate) capable hardware.

The newer specification improves device setup with the “Secure Simple Pairing” mechanism and implements “Extended Inquiry Response,” which reduces power consumption in low-power mode.

For summary, come on apple you can do better than this. Why you discriminate iPhone 3G on your latest OS? As one of the fanboy I am quite dissapointed. Thinking of moving to another platform now.