The thing is, the iPhone 4 did raise the bar compared to the 3GS, IMHO. As a former 3G owner, the 3GS sucked. Overall, the update was ho hum. Faster CPU and compass was the only real update to the iPhone. Any original owners probably were happy with the upgrade, but overall going from a 3G to 3GS, IMHO was entirely useless as it was a relatively boring update.
The jump from 3GS to 4 is bigger. As Jobs put it, it is the biggest leap since the original. Not only is the 4 faster then the 3GS, you get a far superior screen, redesigned case, better battery life, gyroscope, and more ram. The iPhone 4 brought a lot more to the table then the 3GS did.
Jumping from 3G to 3GS was a BIG leap: CPU and double the ram.
The jump from 3GS to 4 is obviously bigger.
But if you say that 3G -> 3GS was entirely useless, well, you don't know what are you speaking about.