Compiled a list of iPhone-specific (but not exclusive) tidbits from the Ars review: https://arstechnica.com/gadgets/2017/09/ios-11-thoroughly-reviewed/
Camera/Photos: HEIF/HEVC capture (A10 and up), QR codes, Live Photo adjustments
Messages: space saving, hide alerts
App offloading
One page, customizable control center
Siri: type to siri, translate, new voice
Scroll to Table 1 at the bottom of this post: https://machinelearning.apple.com/2017/08/06/siri-voices.html
Safari: always on Reader, intelligent tracking protection
Notes: document scanning
Maps: lane guidance, speed limits, indoor maps
Mail: read messages in thread automatically collapsed
AirPlay 2:
Camera/Photos: HEIF/HEVC capture (A10 and up), QR codes, Live Photo adjustments
Messages: space saving, hide alerts
App offloading
One page, customizable control center
Siri: type to siri, translate, new voice
Scroll to Table 1 at the bottom of this post: https://machinelearning.apple.com/2017/08/06/siri-voices.html
Safari: always on Reader, intelligent tracking protection
Notes: document scanning
Maps: lane guidance, speed limits, indoor maps
Mail: read messages in thread automatically collapsed
AirPlay 2:
The biggest change is to the audio bufferAirPlay 2 sends audio out to speakers faster than real time, and the speakers in turn can buffer minutes of audio at a time instead of just a few seconds as they do with AirPlay or plain-old Bluetooth. Obviously, a larger buffer means playback wont be disrupted as often by wireless interference or a device momentarily going out of range.
On Apples end, it looks like all iDevices that get iOS 11, fourth-gen and newer Apple TVs, and any High Sierra Macs that support the original AirPlay protocol should be fully compatible with AirPlay 2. The forthcoming HomePod will support it, too. As for third-party speakers, some manufacturers have already committed to updating some AirPlay speakers to support AirPlay 2 via a firmware update, but it may not be possible for all existing devices; the new buffering system and syncing audio between multiple speakers at once presumably requires some extra memory and other hardware that older speakers may not have."