Keep in mind that this update is intended for developers to get their apps ready for Android P and may be buggy, so you may not want to install the update on your daily driver.
It's simple: Google isn't brining Android P to its earlier devices, including the Huawei-made Nexus 6P, the LG-made Nexus 5X, and the Pixel C tablet. Android P also continues Android's steady improvements for the Autofill Framework and power efficiency (via Doze, App Standby and Background Limits).
Another feature coming with Android P is Wi-Fi Round-Trip-Time (RTT) support.
The current schedule for Android P Developer Preview shows a preview 2 arrival in May (at Google I/O), followed by preview 3 in June, preview 4 in July, preview 5 shortly after, and then a stable release in Q3.
As for what's included in this update, Google has included quite a few new goodies.
All of the changes that Google had detailed today are meant for developers as they will be the ones putting this first Android P developer preview through its paces. You will be able to use Smart Replies, stickers and images directly from within a notification when responding.
The new OS, which we've learned will support display notches and give developers more freedom to harness dual cameras. Good news for those people, because we now know when the final Android P release is coming. Over at Droid-Life, they've taken the liberty of ripping the Pixel Launcher that comes installed in the developer preview and releasing it to the World Wide Web to enjoy.