Humans aren’t as sensitive to improvements as well as are with regression.
My upgrade from the original iPhone to the iPhone 3GS was a huge upgrade in speed. From 3GS to 4 didn’t seem like much difference in terms of speed or the new display. So little that I even started to doubt my choice of upgrade.
Fast forward to today when I did some testing on a 3GS model. Not only was it obviously slower, but I had got so accustomed to the “retinal display” the 3GS screen just looked tacky. I am now using the 4S and it doesn’t seem much faster.
Maybe the transitions are a little bit more buttery but not too much. However I am sure that a year from now if I got to use a 4 again I will probably be able to clearly pick out all the places where it doesn’t does worst than the 4S.