There are a few relatively minor design changes in iOS 9 including the typeface and the shift key on the keyboard.
The Helvetica Neue font, which was introduced with iOS 7, will be replaced with San Francisco, the same font Apple used for the Apple Watch. The differences are subtle, with the letters in San Francisco being a bit sharper when viewed in a small size, and there is more breathing room between characters. San Francisco will be the default font on OS X El Capitan as well.The shift key on the keyboard has changed as well. In iOS 8 it simply changes shades when tapped, and unless you have made a point to memorize which one is which, you probably don't know if it is engaged or not. In iOS 9 the whole keyboard changes - when shift is on, the buttons will show capital letters, lower-case when it's not. Another new feature of the keyboard allows you to bring up a cursor by tapping and holding two fingers on the screen. The keyboard grays out until you've moved your cursor to a new position.