Apple shipped their first batch of iPhone X's with an outdated and unpatched version of iOS. Be sure to check for updates and install iOS 11.1 first thing after you open your new iPhone or it'll be susceptible to the KRACK vulnerability for wireless connectivity.
KRACK, or Key Reinstalltion Attack, allows intruders to decrypt your internet traffic via a vulnerability in devices connected to WPA2 networks. These networks are the most commonly used form of wireless security, so chances are you're going to connect to at least one with your new iPhone X. You can read more about protecting yourself and your other devices from the KRACK vulnerability over at Null Byte.
Aside from security, you're going to want to update to iOS 11.1 on your iPhone X anyway, which comes preinstalled with iOS 11.0.1 — Apple has included 70+ new and improved emojis, labels for Control Center, and more. Obviously, security is the most important reason to update, but come on — you always need the newest emojis.