My wife traveled DCA-JFK-ATH-JFK-DCA a couple of years ago in DL coach and reported that she had good flights in both directions.

I have flown DL transatlantic (the longest flight was MUC-ATL) in both cabins over the years, and have been generally pleased with the service, even in coach. DL's flights were really good in the early 90s, as they featured veteran FAs from the old Pan Am, who knew how to serve a flight and be friendly and helpful (I recall vividly having about a 2 hour conversation with a 40-ish female FA, a 20 year vet of PAA, who was a native of Uruguay, over the Atlantic between JFK and FCO).