Patch 2.1 will have a Public Test Realm (PTR). It is unknown when the PTR or the patch itself will become available. However, note that the PTR will initially not have all the changes for 2.1 on it.
Main features
Seasons
* Seasons or Ladders, as they were known in Diablo II, will be a new game mode to choose from.
*When creating a character, you can pick between Season and non-Season, giving you a total of four options on character creation: Softcore Season, Softcore non-Season, Hardcore Season, Hardcore non-Season.
*Existing characters will not be able to play in the Seasons.
*When choosing to create a Season character, you will be off to a completely fresh start: No gold, no Paragon Levels, empty stash and no (levelled) artisans. You will also be unable to play with any non-Ladder characters. The separation is comparable to that between Hardcore and Softcore right now.
*In Diablo II, you would compete with other players to reach the maximum level faster than them. The highest level players would be listed on a leaderboard, the "Ladder".
*What the measure for Seasons will be in Diablo III is still unclear. Season specific Leaderboards for the new Tiered Rifts have been confirmed but it is certainly possible that the highest (Paragon) Level players will also be listed somewhere. It is likely, that the Leaderboards will be class rather than account based.
*Seasons will only have a limited duration, though it is still unclear how long they will be. The developers have mentioned talking about a time ranging from 1 to 6 months, also noting that the first Season may be longer than average. Future seasons may vary in length. Later Seasons have also been mentioned to possibly change some basic rules of the game, though no examples were given.
*After the end of a Season, all characters, Paragon Experience, gold and stash items will be rolled over to the non-Season game, allowing you to continue playing your Season characters in the other game mode, while providing yet another fresh start for the next Season.
*Seasons will feature exclusive Legendary items as an additional incentive to
play them.
*After a Season ends, these items will become part of the loot table in the non-Season game, allowing all players access to them.
Tiered Rifts
*Tiered Rifts will be a different form of Nephalem Rifts in Adventure Mode.
*The idea behind them is to have an endless dungeon of sorts with increasing difficulty the deeper you go. Each Tiered Rift will be on a timer which you will have to beat to be able to advance to the next level. While the time you have to clear a Tiered Rift has not yet been determined, 15 minutes have been mentioned as an example.
*To access a new Tiered Rift you will have to acquire a Greater Rift Token, which has a chance to drop from the Rift Guardian in a Nephalem Rift.
If you manage to beat a Tiered Rift level in time, you will get a Greater Rift Key of the rank of the next level you may enter.
*There will be leaderboards associated with Tiered Rifts, allowing you to compare how deep you could go or how fast you cleared a Rift tier with your friends, Clan or the entire region.
*There will be separate leaderboards for Hardcore/Softcore and Season/Non-Season as well as different classes and group sizes.
Rewards for Tiered Rifts are still being worked on but they should be the most rewarding thing you can do on a certain difficulty.
*Legendary Gems are being considered as a potential reward.
*In the future, Tiered Rifts may be used to measure class balance in the form of solo capabilities, group synergy or high vs. low difficulty farming speed, allowing the developers to identify possible issues by looking at a large data set: the leaderboards.
Nephalem Rifts
*The time it takes to clear a Nephalem Rift will become less dependent on the Rift's contents. This is in regards to Tiered Rifts and allowing better comparability between the time it takes to clear them.
*Rift Guardians will get some tweaks to make their fights last approximately equally long, e.g. Orlash should disappear for a shorter time mid-fight.
*The general monster density in Rifts will become more uniform and the speed at which the Rift's progress bar fills will be dependent on the healthpool of slain monsters, making Elite packs worth it even if you're going for pure speed.
Itemization
*The following Sets will get their full Set bonuses changed:
-Firebird’s Finery (Wizard)
-Raiment of a Thousand Storms (Monk)
-Monkey King’s Garb (Monk)
-Thorns of the Invoker (any class, Thorns)
-The Shadow’s Mantle (Demon Hunter)
*These changes are intended to affect items found before the patch as well, so don't salvage them just yet!
*Other Legendaries, specifically Monk related ones, might also get improved, but don't expect every lackluster Legendary to receive a buff.
*There will be new, Season exclusive Legendaries which will become available in the non-Season game after a Season ends.
*Legendary Gems were mentioned as possible rewards for Tiered Rifts.
Classes
*While no details are known yet, "many changes" are planned for all classes.
*The Monk changes will improve core class attributes and class abilities.