"The new single sign-on functionality allows players to log in once to access World of Warcraft, StarCraft II, Diablo III, and the upcoming PC and Mac versions of Hearthstone: Heroes of Warcraft, and stay logged in for up to 30 days.
Players can also install games directly through the app and, if you choose to leave the app running on your desktop, keep them up-to-date automatically even while you’re away from your computer. Please note that it’s entirely up to you whether you wish to leave the app running—you can exit it at any time, and it will automatically re-launch whenever your standard launchers would. "
If you want to read more, check out the Blizzard post:
http://us.battle.net//d3/en/blog/10626946#best
If you just want to download the new client for testing:
https://us.battle.net/account/download/index.xml?show=bnetapp