Working with the Photos app is easier
The Photos app is packed with improvements that sound small but which make it a whole lot better to use. For starters, when you're viewing an individual photo full-screen, you don't need to reach for the back arrow at the top-left corner of the screen – a stretch if you're using one of Apple's 4.7-inch or 5.5-inch smartphones. Just swipe downwards to dismiss the photo.
To select many consecutive photos, tap Select at the top-right of a view, as you would in iOS 8, but rather than tapping each photo you want to select, tap and hold on the first one, then drag your finger to the last one and let go. Of course, you can then tap any pictures in that range that you don't want to process to deselect them.
You can now hide all photos in a multiple selection at once. The Hide option has moved from the old tap-and-hold options bar to the Share button; tap that and you'll find Hide in the bottom row of the sheet.
There are two new albums whose contents are automatically maintained: Screenshots and Selfies. The former includes shots taken by pressing the Home and sleep/wake buttons on your iPhone or, if you have an Apple Watch, by pressing the Digital Crown and the side button on that device. The Selfies album, however, doesn't do anything smart like face detection; it merely contains any photos taken with the front-facing camera.