New and old players alike will benefit greatly from the "What has changed?" tab. Simply open up your spellbook and click on the last tab at the bottom. You'll be greeted with a new screen giving you the run down on some of the biggest changes to your class coming in Mists of Pandaria.
On a warrior, for instance, you can see how Blizzard wants returning players to acclimate quickly into the new Mists of Pandaria mindset. Blizzard is letting warriors know that a bunch of their skills might seem missing but have just become baseline specialization abilities rather than something in your book to throw you off. The other big stress points are touched on. Rage generation's mechanic has changed, and stances no longer control what abilities you can use.
The "What has changed?" window is not currently implemented for all classes, and most are unfinished as of this build in the beta. However, I think it's an important enough new UI piece that players who are thinking about returning would get some real use out of. It's interesting to watch the nature of player retention almost eight years down the road for a game as large as World of Warcraft, because so much has had to change in order to get to the point that we've had almost three complete class overhauls since the game launched.
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!