While there aren't any headline-grabbing new features here (those are surely being held back for Google I/O later this year), Android 5.1 does have a few new goodies in store, including support for multiple SIM cards and HD voice, and better protection for lost or stolen devices.
Multi-SIM phones are practically unheard of in the US, but are much more popular in Asian countries. With the 5.1 update, Android now supports dual (or tri-, etc.) SIM phones natively, without any third-party OEMs add-ons needed.
Android 5.1 also brings "Device Protection," which, if you lose your device, will lock it out to anyone who doesn't have your Google login info – even after performing a factory reset.
Native HD voice calling is also added in Android 5.1, offering higher quality voice calls (yes, your phone can still make those) – as long as both parties are using compatible phones and carriers.
The 5.1 update also includes performance and stability improvements, as well as minor tweaks like the ability to join a Wi-Fi network or control Bluetooth-paired devices straight from the Quick Settings pull-down menu.
As always, Nexus devices owners should see the update first, with phones that have manufacturer skins (like those from Samsung, HTC, LG and so on) likely coming much later on.