The "back" button. I love that if a link or notification sends you to another app from the app you're in the target app has a "back" button to take you back to where you were. use case: You're reading an e-mail which contain a link. you click the link and Safari opens the site. Once finished you can click the "back" button that appears at the top left to go back to your e-mail. No need to use the task switcher or to press home and tap the mail icon again (for those who dont use the task switcher). The back button also appears if you trigger another app through the notification center or from a notification banner. Great little feature in iOS 9.