The most notable changes for many are the frameworks. High Sierra moves to the faster, more reliable Apple File System for storage (much as on your iOS devices), and the Metal 2 graphics interface offers both better performance as well as support for technologies like machine learning and virtual reality. You'll also get support for HEVC (aka H.265) video if you have a Mac with a 6th-generation or newer Intel Core processor.
Apart from that, most of the changes are tweaks. The Photos app gets a big revamp with more editing tools, an expanded interface and support for iOS-like features such as Live Photos and Memories. Siri both has a more natural-sounding voice and the ability to learn your Apple Music preferences. Spotlight now includes flight details, and Safari is fighting back against web annoyances through enhanced anti-tracking protection and blocking for auto-playing videos. You'll probably appreciate Apple's OS nip-and-tuck upgrades, then, but you won't be bowled over in the short term.