Gestures on iPad 1.
AirPlay mirror on iPhone 4
Siri on iPhone 4
AirPlay on first gen core i7 machines.
iCloud on Snow Leopard. Maybe bad example.
iBooks on SL.
But AirPlay on gen 1 i7 gets me most since intel itself says it is supported.
There are more but These jump to mind.
Gestures on iPad 1. - Only pinch to home screen works well in the iPad 1, application switch with 4 finger is slowwwww. But apple backed their decision and gestures are now available for the iPad 1
AirPlay mirror on iPhone 4 - probably the CPU is not strong enought to do that without compromising battery life/ performance
Siri on iPhone 4 - same as before but regarding the silent noise chip the iPhone 4s has
AirPlay on first gen core i7 machines. - you don't want your 4(8) cores at full throttle to encode the video that goes to AirPlay Mirror.
iCloud on Snow Leopard. Maybe bad example. - You are right, this was a greedy/ lazy move
iBooks on SL. - Apple probably thinks that a computer is no good for reading books, I also think that.
There is an explanation for almost everything