There are a few relatively minor design changes in iOS 9 including the typeface and the shift key on the keyboard.
It has long been a frustration of iPhone owners, especially those of 8GB and 16GB models, to see a sizable chunk of their device's storage devoted to the operating system. Thanks to a new technology known as "App Thinning" that will be introduced with iOS 9, this will no longer be the case. As noted in this week's WWDC Keynote, the technology was used to pare down the iOS 9 OTA update from 4.6GB to 1.3GB, a 71% size reduction.
Apple announced at WWDC 2015 that iOS 9 public beta would launch in July. For those with older devices, the good news is that any hardware capable of running iOS 8 will also be supported by iOS 9. This means that when the firmware officially releases in the fall, the following devices will be compatible with the update: