iPhone -> iPhone 3G: Addition of 3G, GPS. enclosure became plastic.
iPhone 3G -> iPhone 3GS: Video camera
iPhone 3G -> iPhone 4: Retina, significantly improved camera, new enclosure, addition of front facing camera
iPhone 4 -> iPhone 4S: Siri, better camera
iPhone 4S -> iPhone 5: New screen form factor, significantly improved enclosure, lightning connector, LTE
iPad -> iPad 2: thinner and lighter, cameras
iPad 2 -> iPad 3: LTE, Retina
iPad 3 -> iPad 4: lightning connector
I don't see any significant decline in diff quality over time.
edited for correctness
The 4S, while having a better camera, it wasn't a huge jump, and Siri was software they removed(which had more features) from the 4's app store.
The 4 was the last truly innovative iPhone- no one was talking screen density, and they nailed it. The new camera was a gigantic step up, the front facing camera was as well.
Also, you missed with the iPhone->3G that the App Store came into existence.
As to the iPad, the iPad 2 was commonly referred to the moment it came out as "The iPad that Apple meant to release." because of all of the small, but numerous, improvements. Adding Retina to the 3 is arguably a similar jump, though admittedly I don't have a lot of room to really know, as I don't own one.