The big upgrade is that, with an ATV2 or ATV3, you can finally use a Bluetooth keyboard with it. If, like me, you've spent far too much time searching for shows by the "hunt and peck" method of typing on a remote, this may be the best upgrade ever.
Apple will only guarantee that this will work only with its own Apple Wireless Keyboard, but I found it worked just fine with my Logitech Bluetooth Easy-Switch Keyboard. It was a real pleasure to finally be able to touch-type my way to the shows I wanted.
In addition, you can now send audio from your Apple TV to AirPlay-enabled devices. This isn't a big deal for everyone, but I like that I can now throw my movie or TV show's audio to my Airport Express-equipped house stereo system.
Finally, as far as new features go, Apple TV 5.2 also supports iTunes in the Cloud. This will let you play any videos you've purchased from the iTunes store even if your media server is on the fritz. Oddly, though, you can't use it to play music from iTunes in the Cloud.
As usual, the upgrade also included security and stability fixes. Put it all together and you get, for me anyway, a game-changer of an update. Want to give it a try, just follow the instructions and you'll be using the new and improved Apple TV operating system in minutes.