Most notable are the addition of Greater Rifts and Legendary Gems. Unlike Nephalem Rifts, Greater Rifts force players to slay enough monsters to summon a Rift Guardian while racing against a countdown clock. Unfortunately, the only creature in the Greater Rift capable of dropping any items is the Rift Guardian, so if you don't kill enough demons to summon the big fella, you've wasted all that time and effort. As you'd expect, those who do manage to kill the Rift Guardian within the time limit will walk away with some very nice rewards.
Legendary Gems offer all the benefits of their standard counterparts, but can be infinitely upgraded and include unique enhancements not found elsewhere. They can only be socketed into rings and amulets, but each time you complete a Greater Rift there's a chance your Legendary Gem will be upgraded, depending on how quickly and decisively you dispatched the Rift's denizens.
Full details on what's included in Patch 2.1.0 can be found on Blizzard's Diablo 3 website. There's currently no word on when or if this new content will be added to the recently-released Diablo 3: Ultimate Evil Edition, but Blizzard has committed to supporting both the Xbox One and PlayStation 4 versions of the game with post-launch content additions.