Animoji -- Custom Animated Emoji
It's a relatively small feature, but it is one that most users will love. The same camera that makes Face ID possible also allows you to make custom animated emoji from a dozen different emoji. This is built into Messages and you will be able to send these to other people even if they don't have the iPhone X. But you need the iPhone X to make them.
You can change the expression and you can record audio and movement for these animated emoji, allowing you to really take your Emoji game to another level. It looks like this is limited to the Messages app for now, but you can likely send these to Android users too. For now you can only pick from 12 emoji to animate, but we expect that this will grow over time.