2. New Photo and Video Format
Apple has switched the format for photos and videos. It now uses HEIF format for videos and HEVC for videos. They’re efficient when it comes to compression algorithms. But you might be facing issues when recording video or when transferring photos from your iOS device to your PC.
You can switch back to the old formats by going to Settings -> Camera -> Format. Switch to Most Compatible here.
To make sure there aren’t any issues when transferring photos to the Mac, go to Settings -> Photos and from the Transfer to Mac or PC section, make sure Automatic is selected.
3. 32-Bit Apps Won’t Work
iOS 11 stops support for 32-bit apps. There’s no way around it. The apps just won’t launch. To see a list of all the 32-bit apps you have on your device, go to Settings -> General -> About -> Applications.
If you just can’t live without an app listed here, you can try downgrading back to iOS 10. But it will better if you try to find an alternative to the app on the App Store.