I hope iOS 9 is 80-90% bug fixes and performance improvements. Make the UI perform smoothly again, fix the quirks like copy and paste being broken, battery percentage skipping 30% and 60%, etc, etc, etc.
10-20% new features, useful things. Dark mode. Preferences pertaining to where to have translucency, where not to have it, open up quick reply API to third parties, split screen multitasking on iPad, make the iPad more distinctive from iPhone, but KEEP IT SMOOTH.
And on top of that a more refined UI, or a little refresh like how iOS 6 changed the settings icon, made little changes to the way headers look and the status bar inside of apps, just made things look more pretty, less stale, yet not affect performance. Maybe they could even improve performance by doing this. Like taking the translucency out of places where people would never even notice it. Any little thing like that can help. They need to pay attention to detail again.
I don't know if I'm asking for too much but I just don't want my devices stuttering so much in terms of UI performance. My 1 year old iPad mini 2 feels 2 or 3 years old, and there's been UI stuttering from the beginning (CC, NC, Siri, etc + keyboard or lock screen has always stuttered)
There's even stuttering on occasion on my iPhone 6, the newest iOS device there is. I remember I was jealous of my friend's iPad 3 back when it had iOS 6, it was a year old. Everything ran perfectly. The iPad 2 was 2 years old and it ran iOS 6 perfectly as well. Mini 1 was perfect, iPod 5 was perfect, iPhone 4S was perfect... They were on dated hardware. Felt almost impossible to slow down those things. Only time I remember UI stutter was when closing an intensive game out of nowhere, the exit animation dropped frames, depending on the situation. But that was understandable. Now, I can instantly think of anything to slow down any iOS 8 device to date. Things like this bug me, when using the pinch to close gesture on iPad, there's stutter as you are closing an app if the keyboard is open. If there's no keyboard, it's fine. Usually. Why? This makes no sense. Control Center and Notification Center stutter on iPad just because the keyboard is open as well. Why?? I'm sick of seeing this lag related to the keyboard. Everything is twice as bad with a split keyboard.
Maybe I'm only remembering the good things of iOS 6, but as far as I'm concerned, 99% of it was 60FPS. On a 1 year old device. I really want that great performance back, it's one of the only things I miss about iOS 6. I think many people agree, and that's why people are hoping iOS 9 fixes everything.