Don't know about others, but Delta's new system is based on MQM (Medallion Qualifying Miles). The number of miles differs on different SkyTeam partners in different classes of service. For example, DL/AF/AZ business class is double MQMs.
In the Delta program, 1 million MQMs gets you Silver Medallion for life; 2 million MQMs gets you Gold Medallion for life; 4 million MQMs gets you Platinum (highest status) for life.
The question is whether Silver or Gold has any specific advantages over being a non-elite ... and that is debated vociferously in the DL group. Platinum has one benefit that is real nice - free membership in the Crown Room Club. However, other parts of the SkyMiles program is undergoing change. You might look in the DL group for specific discussions.