Continuity, including Handoff, Universal Clipboard, Cellular Calls, SMS/MMS, Instant Hotspot, Auto Unlock – requirements and how to set each up. Note that most need both Bluetooth and WiFi to be turned on: if your Mac only has wired Ethernet enabled, features will not work fully (or at all).
Sierra Help online – the same content as is delivered by the Help menu.
Using Siri on your Mac.
Optimised Storage – new features to store files in iCloud, and perform housekeeping accessed through About This Mac, in the Storage tab.
Supported printers, scanners, and faxes – although this list does not yet specify which are supported in Sierra, the implication is that those listed are.
Mac models officially supported by Sierra. If you want to try using Sierra on a model not listed here, try macOS Sierra Patcher Tool.