If you buy a blue, purple or metallic copper Galaxy Note 9, you're able to write notes on the "off screen" in a signature color -- this is brand new to the Note 9. (The blue model writes in yellow, the other two write in purple and copper, and the black Note 9 writes in white).
You're not limited to just writing and navigating with the Note 9's S Pen. You can also use the button as a remote to do different things in different apps. For example, pause and play a YouTube video.
Introduced in the Galaxy Note 8, Live Messages are basically animated GIFs you can make and share with friends. You can access it from the Air Command wheel and through the Samsung Messages app. They'll work with any sharing platform that supports GIFs.
Brand new to the Note 9 is making a Live Message with an animated GIF of an avatar you make with the AR Emoji tool inside the phone's native camera. AR Emoji was introduced in the Galaxy S9 and S9 Plus. I'm not personally a fan, but it's here if you want it.
One of my favorite ever uses of the S Pen is in taking precise screenshots of only the part of the screen you want. Every other phone requires you to screenshot the entire display and then crop down. This is a thousand times easier.
The S Pen has nothing to do with this trick, but I'm partial to it anyway. Instead of receiving a pop-up notification, you can opt for the perimeter of the phone face to light up in a variety of color choices every time you get a message. It's a little disco, but who cares? Go to Settings > Display > Edge Screen > Edge Lighting > Edge Lighting style.