Siri for Mac (pressing and holding Fn key, with typing support for no-voice option)
Focus mode (blurs and/or dims space around foremost app)
Improved Dark mode which actually darkens window chrome, buttons
All default Mac apps rewritten in Swift
New Control Center on left side of screen which replaces Apple Menu and relocates some menubar icons like Bluetooth, Volume and Time Machine Bartender style, as well as housing all Notification Center widgets
Clipboard history in Control Center to retrieve text, images or any recently copied file with ease
Simplified UI for Notification Centre, simply showing Notifications like iOS
A dedicated Music app for Mac for Apple Music, iTunes discontinued but remains an option
The News app comes to OS X, syncs content and reading position with iDevices
iSync app for handling all iDevices, Watches and general syncing including between other Macs
3D dock option returns in Appearance Settings, remains frosted/flat but reinstates some depth to UI
Generally refreshed icons across the OS, making them more detailed and rich/deep in colour while still flat (feature parity with iOS 10)
Fl.ux like feature which optimises display at sunset/nighttime so it's less stark, bad for eyes (feature parity with iOS 10)
More control over Do Not Disturb scheduling (feature parity with iOS 10)
Ability to Snooze Mails and Messages to deal with later (feature parity with iOS 10)
Messages app updated to include live previews of content like webpages, large emojis (feature parity with iOS 10)
Per-contact read receipts in Messages (feature parity with iOS 10)
Voicemail for FaceTime (feature parity with iOS 10)
iCloud Backup for Mac for those with paid storage tiers, granular control of what is backed up
Wider iCloud Drive support, ability to choose folders and files to mirror in iCloud Drive for access on other devices
Force Touch support in Launchpad for shortcuts on icons, like iOS home screen
See my iOS 10 wishlist post.