Delta and their SkyMiles program is one of the most popular mileage programs in the United States. They have been at the forefront of negative innovation in the mileage program game. At first their program was similar to others that gave you miles based on distance but they added a spend component called Medallion Qualifying Dollars or MQDs back in 2013.
They were also the first to remove the award charts so in effect you do not know the exact cost of a mileage award up until the moment that you enter on the delta.com website.
The flip side of it is that they are very lucrative when you credit partner airlines especially in business and first class so this allows you to collect a significant amount of redeemable miles and spend at a lower cost compared to a similar Delta flight.
You can earn Delta MQMs, RDMs and MQDs from at least 403 unique airfare buckets across 4 types of cabin (First Class, Business Class, Premium Economy and Economy) and subcategories within each area on over 20 different airlines!!
There is a huge range of earning opportunities from 200% MQM, 300% RDMs and 60% of distance as MQDs on the high-end to as little as 0 MQMs, RDMs and MQDs!