The folks at 9to5Mac have detailed what to expect in macOS Sierra, and the list includes a set of very useful features.
Once your Mac is backed up and your data is safe if anything goes wrong, it’s time to update. Apple says these Macs are compatible with macOS Sierra:
MacBook (Late 2009 or newer)
MacBook Pro (Mid 2010 or newer)
MacBook Air (Late 2010 or newer)
Mac mini (Mid 2010 or newer)
iMac (Late 2009 or newer)
Mac Pro (Mid 2010 or newer)
As for feature requirements, you may want to check this page to see if a feature that you want works with your Mac. Auto-Unlock, for example, requires 2013 or new Macs due to Bluetooth limitations so my 2012 Mac mini doesn’t make the cut.
What continues to amaze me is that the vast majority of the new tweaks run perfectly on my mid-2011 iMac and that a brand new OS can run so smoothly on a 5 year old machine. It puts the ‘cost’ of a new Apple computer into perspective.