The DL lounges I have been to are OK and better than AA/UA in general, with the exception of the AA lounge I got into at JFK last week thanks to my status and ticket.

I am not comparing DL and Qatar in general but when long- haul premium classes are concerned I think DL has a lot of catching up to do. DL's prices are right up there with the Gulf cariers in many markets and the DL suites are second to none in my opinion. They just need to put more in their premium experience, like AA has done at JFK, and AC with their premium lounge, only open to paid business and Flex rewards.