The pundits are saying it's for real this time. They do have some good complements. Delta is big in Europe (especially FRA) and Northwest is big in Asia (especially NRT). The are also part of the skyteam alliance.
What to do first? Complete the shrinkage of the MEM hub (covered by CVG and ATL) and the retirement of the DC-9's. What about the other hubs? DL already pulled out of DFW. I'm not sure they will shrink any other hubs. Looking at a map maybe CVG could be a candidate for the axe.
Problems: besides the pilot's agreement, despite the great diversity in DL and NW's fleets, they have almost nothing in common. NW's old planes are DC-9's, 747's and they are about half way to a very slow process of converting to Airbus. Delta's old planes are MD-80's and they are much further along to Boeing's new planes. The only thing they have in common is the discontinued 757-200.