mobile

1. Double tap home button to launch the camera in under one second.
1B. Disable the double-tap quick launch in the camera options to get rid of the delay when pressing the home button. It's a neat feature, but makes the home button much more quicker. (Thanks to @B1aze)
2. Use themes to change the look of your phone (limited as of right now; will be more widely available in the coming weeks to developers/themers).
3. Go into your App drawer and long press an icon as if you want to move/delete it. The small circles with the red '-' sign will let you either delete an app (if third-party) or disable it (if built-in).
4. Hold the power key for more than 7 seconds and the device will reboot.
5. Whenever you want to take a "selfie", you can simply frame the image first, and then put your finger on the heart rate sensor under the LED light at the back in order to take one.
6. When downloading a large file (30+ MB) and you have LTE AND are near WiFi, activate Download Booster! Settings--->More Connection Settings--->Download Booster. It combines both your WiFi and LTE connections into one for amazing DL speeds.
7. Notification reminders! Settings--->Accessibility--->Notification reminders.
8. Don't forget to claim your free 100 GB cloud storage with OneDrive!
9. Private mode... You can hide content from prying eyes, including photos, videos, music, web history, and such. To activate it, go to Settings--->Privacy and safety--->Private mode.
10. Do Not Disturb mode. Dare I say... Better than Google's implementation of "priority notification only" mode. You can set a schedule for it, who IS/ISN'T allowed to contact you while it's on, etc.
11. Screen Pinning. Go to Lock Screen and Security--->Other Security Settings--->Pin Windows.
12. Customize your screen grid (4x4, 4x5 or 5x5) by pinching in when on the home screen.
13. Developer Options.. To enable this, go to Settings--->About Device--->Tap "Build Number" 10 times and you will unlock Developer Options!
14. Make your device a bit faster by going to Developer Options and setting window animation scale, transition animation scale & animator duration scale all to 0.5.
15. when you are in an app (Chrome, Gallery .... whatever) and you drag your finger from the upper right corner to somewhere in the middle of the screen the app shrinks which makes it more usable with one hand. (Thanks to @derWolf_)

Will be adding as time goes on, feel free to add your own and I will add it to the OP!