7. Mute audio playing in Safari tabs.
Safari maintains a track of the tabs that have audio playing. Ability to mute the audio playing in other tab is one of the Unknown features of El Capitan. Just click the small speaker icon at the right of the tab. You can unmute it by clicking on it again.
8. When all the Apple Devices coordinate.
You get a coordinated alert if you’re sitting with your iPhone and iPad lying nearby. Though it was more of a cacophony earlier. In Sierra and iOS 10, alert only appears on the device you’re using.
9. Slow-Mo effect.
Hold shift while minimizing a window ( Works with most of default OS X animations ) and watch the animation unfold. Not of any use but it surely manages to grab your attention each time.