Search in Settings

This one is a total painkiller! It was always a mess trying to find that specific setting and now all you need to do is type in.

Low Power Mode

This feature is also another big deal in terms of usability. More often than you would expect, the iPhone starts to drain battery like there's no tomorrow. In those situations that you still need it alive (for an emergency call, etc), the Low Power Mode will get you to shore.

What it does is stop any background requests like Mail fetch, Push Notifications, Apps Background Refresh, etc and also visual effects (as if Reduced Motion was on).

A button to go back to the last app you were in

Any time you're in one app and then suddenly you're in another app, without going through the Home screen or the App Switcher (such as if you tap on a push notification, or if an app deliberately sends you to another app like Facebook opening Messenger), you'll get a Back button.

Siri Integrations

For me, and for everyone in general I guess, the big reason to update to iOS 9 comes in the form of a Siri request. Siri got a lot smarter from the previous iOS (she is still a bit stupid, but lets cut her some slack).

Siri now as access to almost all the information stored on our devices. For instance, if you receive an email with a phone number on its signature and later on you receive a phone call form that number, Siri will make an educated guess and suggest that the person calling you is the same from that email signature.

As for me, the major leap in terms of Siri usage for automate things and really getting things done more easily is the ability to ask her to remind you of anything, anywhere on the OS in the most natural way.

As you can see in the image, I asked her to remind me of what I was seeing on Safari when I got to work. She created a reminder with that action attached to it.

Spotlight quick conversions and calculations

You can now do a quick calculation on the Spotlight menu, just the basic stuff though. No square roots i.e.

Detected audio connection

In iOS 9 everytime you connect your headphones to the device, it will automatically show you a shortcut for the last used app with audio output (Music App, Spotify, Audible, etc).

If you are on the lock screen, it will show up on the low left side and if you are multitasking it will appear right at the bottom of the screen.

This is actually another great feature in terms of productivity: when you have a Handoff capable app opened on 2 devices, the handoff shortcut on the iPhone is now always visible at the bottom of the Multitask Screen (whereas on iOS 8 it was at the very left end of the opened apps slider).

New San Francisco Font

My favorite part of iOS 9 is the new San Francisco font. It really makes iOS 9 seem a lot newer and different than iOS 8. I think it looks way better than the old font.

Spotlight search integration with installed apps

Developers can now take advantage of Spotlight search to ease overall search on iOS 9. In this example: Citymapper showing recent searches I made in the app for Lisbon

Battery Widget in Notification Center

If you use your iDevice with a pair of bluetooth headphones or you have an Apple Watch, this widget might come in handy.

It basically lists the battery levels of every peripheral connected to the device.

Ability to choose video resolution and frame rate

A pretty great thing in iOS 9 is the added ability to change the resolution and frame rate in videos that one shoots.

Just go to Settings -> Photos & Camera -> Record Video. The higher the resolution and frame rate, the better the video looks, but the more space it takes up. Those with 8 and 16 GB iDevices should be especially happy about this one.

Double-click the home button while in sleep mode for Apple Pay

(For iPhone 6 and later)

You no longer have to wait for the NFC reader to wake up the phone, or manually navigate to the Passbook/Wallet app, to prepare the phone for an Apple Pay authorization.

Simply take the phone out of your pocket and double-click the home button. You can then authorize immediately, or select a card first if you have multiple cards loaded into Apple Pay.

When typing with the keyboard, you can convert it to a trackpad by placing two fingers on the screen anywhere inside the keyboard. The keys will turn dark gray, losing all letters and numbers. You can move the cursor by dragging both fingers across the screen, lifting your fingers when the cursor has reached its destination.

One thing I think is in there but hasn't already been mentioned:

Ability to use Spotlight to search within participating third-party apps

Let's say, as an example, you have a bunch of recipe apps on your phone and you want to make a delicious fettuccini alfredo dish for dinner. All you have to do is do a Spotlight search for "fettuccini alfredo" and the iPhone comes up with a list of recipes from the various recipe apps.

Small tweaks to the “Photos” app.

For me, the fact that the “Photos” app has some seemingly minor—but noticeable—kinks worked out has been great. Most notably, now you can swipe across a grid of images to select multiple images instead of having to select each image one by one.

Also, there is now a “Screenshots” album which seems to be a “smart” album that only shows up when you take a screenshot. But unlike other albums where deleting an image in an album only removes it from the album, the “Screenshots” lets you delete screenshots completely from your device.

This is a quite nice way of dividing up content that makes it easier to manage screenshots when mixed in the context of “normal” photos.

And past that the order of images when viewing via “Photos” or via “Albums” is now exactly the same.

Prior to iOS 9, if you viewed photos in the “Albums” view it would be oldest to newest from left to right, but in “Photos” it would oddly be newest to oldest from left to right.

Now with iOS 9, both “Photos” and “Albums” show oldest to newest from left to right.

Not much of a feature as it is a help for developers and app performance:

Metal API support with SceneKit.

By default, SceneKit games will render with Metal on Metal-capable phones. On iOS 8, SceneKit used OpenGL ES. Some things are different with iOS 9 and iOS 8 SceneKit as a result. For example, if you render a 3D scene in iOS 8, OpenGL ES renders 2D content in front of the 3D content, ignoring depth of field.

However, with Metal, I'm not sure where the 2D scene is positioned, and it renders depth of field based on texture. Using it on labels looks awkward at best:

It doesn't blur in the box of the texture, so 3D nodes "behind" it don't blur, causing a box to appear around 2D text.

I'm sure 2D overlay scenes have a z position relative to the 3D scene, I'm not quite sure what that is yet. Maybe 0? Apple has no documentation for that.