Messages Upgrades in macOS Sierra
Just like with iOS 10, Messages gets a facelift in macOS Sierra and also gains new features.
Emoji are now bigger in the transcript than on OS X El Capitan and you can send tapbacks, small reactions to messages without typing something new. This is like a Facebook reaction in a way.
Videos now play in inline and you can see link previews inside conversations. It does not look like Apple is adding in all the iOS 10 messages features to macOS Sierra yet, which is not surprising.