The arrival of Siri on the desktop is the standout feature, as was widely rumored. You can speak out queries to Apple's voice assistant to find files on your laptop (using natural language search), play music, search the web, send messages to your contacts and all the other features available on iOS.
Siri gets its own icon in the dock and integrates neatly with the Notification Center too.
The other changes heading to Apple's desktop software are less dramatic but still useful. They include an Auto Unlock feature for signing into a computer with an Apple Watch or iPhone (no need to type your password every time) and a Universal Clipboard to copy and paste text between multiple macOS and iOS devices.
iCloud gets a couple of tweaks too. It will soon include the macOS desktop as one of the folders it monitors, so you can get at all the files on your desktop from any device, and there's a new optimized storage feature designed to free up space on your computer by automatically shifting older files into the cloud (a nice touch, but bigger SSDs on MacBooks would be even better).
Apple Pay is coming to the web as well, so you'll be able to use your credentials in a browser just as you can at a terminal in a store. Consider it Apple's move to turn its payment system into a PayPal rival.
Finally for macOS, Apple is adding tabs to all its desktop programs and giving developers the option to do the same, so multiple maps, documents or anything else will appear in tabs rather than separate windows. A new picture-in-picture mode is being added as well, so you can keep an eye on videos while you work in other programs.
Siri is obviously the most exciting new feature coming later this year, but tabbed applications and Universal Clipboard also look helpful. At this stage macOS is a finely tuned piece of software, but Apple continues to make all of its hardware devices work better together as you switch between them. Of course, as a business move, that means it also serves as another incentive to go all-in on Apple's hardware ecosystem.
A developer preview of macOS Sierra is available today with a public beta scheduled for July. The software will roll out fully in the fall (Northern hemisphere), Apple says, and will again be free. MacBooks and iMacs from late 2009 and later will be eligible, as will any MacBook Air, MacBook Pro, Mac mini or Mac Pro from 2010 or later.
Product page: Apple