http://www.diablofans.com/news/47996-confirmed-season-and-ladder-like-system-in-first
http://us.battle.net/d3/en/forum/topic/12081718116?page=4#64
Posted by Lylirra
Nephalem Trials are indeed still alive in some form, but they will not be added with the launch of Reaper of Souls. We absolutely love the idea behind the Trials (challenge-based systems can be so much fun), but before we implement them we want to make sure they have a strong underlying reward structure and tracking system. That kind of hook is important for a system like this and we didn’t want to add in Nephalem Trials to the game without it.
The gameplay felt great and was in a good place at gamescom 2013 where we announced them – which is actually why a lot of the individual Trials have been added on their own as standalone events into Reaper of Souls – but the tracking mechanism that made the overall system feel meaningful over a long period of time wasn’t and would take some pretty significant tech and time to implement (in this case, at the expense of other features as well).
Rather than delay the release of the expansion to make sure Nephalem Trials could make it in with all the necessary components, we made the decision to cut them for now and re-focus our efforts on making sure other key systems like Adventure Mode, Bounties, Nephalem Rifts, and Clans & Communities (just to name a few) are solid. It’s not an ideal scenario, but these are the kinds of hard calls we sometimes have to make in a world where we can’t stack +Maximum Resource. >.>
Edit: For clarity, Nephalem Trials are not Nephalem Rifts. Nephalem Rifts are definitely still in the game!
Source: http://us.battle.net/d3/en/forum/topic/10491039663#7
They also have some code that can be repurposed for that (the score board system used in Team Deathmatch). Ladders/Seasons is somethings that requires integration with Battle.net 2.0. If done properly, it could also be inclusive of Clans (so not only individual PvEvP challenges, but also allow Clan competitions), and also provide more online profile/armory functionality by pulling not only achievements, but ones individual season ranking details (in addition to the overall leaderboard).
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Basically, Seasons is going to cater to many different demographics like those who enjoy re-rolling, achievement hunting (as you can kill two birds with one stone), enjoy cosmetic type of rewards, enjoy challenges, enjoy the competitive aspect of being seen on a leaderboard.
Myself, I’ve always found leveling in Diablo III, more enjoyable/rewarding compared to max level game play (part of that however is due to the lack of stuff to do once you’ve played at max level for awhile). Seasons is the type of system that I’m looking forward to (the devil is still in the details though with the actual implementation), and will give us something to do with the gear that we find.