The riskiest and possibly most interesting part of High Sierra is the new APFS file system, which replaces HFS+ as the new default. In existing betas of High Sierra there’s been an option to stick with HFS+ when installing, but otherwise your drive will be in-place upgraded to APFS.
Other features of High Sierra include the Metal 2 graphics API, which supports VR and external GPUs, and the new HEVC and HEIF video and image formats which have also been added to the upcoming version of iOS.
Some of the handful of user-facing improvements include tweaks to Safari, Photos, and Siri.
See https://www.theverge.com/2017/6/29/15886832/apple-macos-high-sierra-preview-features 
Apple macOS High Sierra preview: the biggest Mac update you’ll never see
There are some new features and app updates, but the key changes are underneath