WhatsApp has had an interesting relationship with Windows Phone. It has disappeared from the Store several times, had major bugs, and been an overall pain to some people. Despite all that it continues to be updated and improved. The latest update for Windows Phone 8 adds many new features and fixes.
First and foremost is lockscreen notifications. With a messenger app this is a very important feature to have. The tile has been redesigned to be much more metro looking. You can now hit enter to send (yes!). There is a whole new attachment screen and changes throughout the whole UI as well. Lastly, it is now possible to backup your messages. Check out the new and improved WhatsApp in the Windows Phone Store for free.