iOS 4 (as well as 3.1 on the iPad) don't just drop a wallpaper behind, there's a large shadow that is drawn bottom to top over the wallpaper, as well as each icon has it's own dropshadow. In addition the wallpaper reflects into the dock.
In addition there are completely new animations for the homescreen.
Jailbroken wallpapers do not do it the same way. Obviously Apple could have just disabled the dropshadow + effects for the 3G but they chose to either enable or disable features that didn't work as intended rather than strip it down.
Exactly! And that's what happens with tech, new features come out on new hardware, if its important to you then you upgrade.