7. Apple Pay Shortcut from Lock Screen
This is the most helpful feature i found in iOS 9. No more need to go through several taps to access Apple Pay for making a payment. You just need to tap the Home button twice and done. Remember, that this feature is only available with and above iPhone 6 and 6S.
8. Change Side Switch Functionality
Dont know about others, but i never used the Side Switch button to mute or enable ringer on my iPhone. So, i dont find it much useful. However, with iOS 9, it is helpful for me since i can change its functionality to enable to disable screen rotation.