Couple of things I found out today about Sierra. First off, I used to get a warning when I ejected my external music drive that it was a partition and would I like to eject the entire drive. That warning has gone so be careful and make sure you eject all partitions before unplugging external drives.
Second, Gatekeeper has been given more teeth. The option to download from anywhere, as opposed to the App Store and trusted developers, has apparently gone. Haven't attempted to d/l anything that might have needed the option yet, but it will be interesting to see how it will work.