Basically all the previous Medallion features are kept in place but now a fourth tier, Diamond has been added to the elite tier levels. The Diamond level will for those who earn 125,000 MQMs or fly 140 segments per calendar year and it will apparently offer the richest benefits of any airline including complimentary lounge membership.
Probably the best new feature is that Delta will be the first airline to offer rollover elite qualification miles to make it easier to reach an elite level in the following year. For example if your earn 40,000 EQM (or in Delta’s case MQM: Medallion Qualifying Miles) in a calender year, the 15,000 extra miles over the 25,000 Silver Medallion requirement will count towards the next years qualifying.
For complete details and feedback from many Delta Elite flyers see this post on FlyerTalk