The Diablo 3 2.4.0 patch has been unleashed upon the Americas, and as promised, it's a biggie. It adds a new forested locale called Greyhollow Island, new additions to the Ruins of Sescheron and Leoric's Manor zones, 24 custom-designed “set dungeons” built around specific class sets, empowered Rifts, Season Rebirths, a revised Buff Bar, and the addition of "Action Combat," which is apparently a popular feature on the console versions of the game, to the PC edition. And those are just the big bits.
The patch also makes a multitude of smaller changes and additions to the game, which you can learn more about by way of the video above. Blizzard noted that the patch is currently only live in the Americas region, so there's no need for alarm if you're not prompted to download it right away. However, if your home region is set to the Americas, you won't be able to log into Europe or Asia until those regions have also been patched. When that will happen, Blizzard did not say.
A detailed breakdown of the Diablo 3 2.4.0 patch can be found on Battle.net.