It's a bit of a hidden feature, but for 4 years, iOS has had multitasking gestures.
http://www.macworld.com/article/1163019/ios_5_new_gestures.h...
If you swipe with four fingers to the left or right, you can move between your two most recent apps. Hopefully iOS9 will make "multi-tasking" even easier.
I find the swipe and pinch gestures essential.
I think I heard at WWDC that iOS 9 adds support for command-tab, which goes great with a hardware keyboard.
*On iPad. This gesture does nothing on my iPhone 5s running iOS 9.
True, it is iPad only. To be fair, it's a bit of a struggle for me to even get 4 fingers on 5s at the same time, let alone swipe them, but it would be handy on my 6+. I use swiping to switch apps on my iPad heavily. It revolutionized how I use the device.
Now that iIOS has multitasking it changes in an important way. I wonder if it can be a productivity device now. But its still not great when it comes to balancing more than 2 apps.