There's never enough time during a WWDC keynote for Apple to enumerate all of the new features it's adding to its latest software, but this change in iOS 9 will be a most welcome and apparent one: the keyboard now shows lowercase letters when typing in lower case and uppercase letters when typing in upper case. Up until now, much like the physical keyboard on your MacBook, all letters were uppercase at all times. If the change seems like an obvious and overdue design tweak, that's because it is. Android keyboards have alternated thusly for many years, helping the user know what mode he or she is typing in without needing to reference the Shift key at the side of the keyboard.
Update: Read the iOS 9 review.
Prior to iOS 7, the Shift key used to be highlighted in blue when deployed, providing a decent visual cue when not obscured by your typing fingers. Apple changed that to a monochromatic highlight, however, and made it rather more ambiguous — all of which is now a non-issue thanks to the change to show lower and upper case letters for what they truly are.
Verge Video: Apple's incremental changes at WWDC 2015