Get bought out by another airline...
I know this is supposed to be for constructive advice...but seriously, I think the only way that Delta can improve is to be bought out by another airline and restructured from the ground up.
Staff is rude (on the phone, at the ticket counter, at the gates, flight attendents, baggage handlers)
Equipment is out of date (old computers, old planes, worn out seats)
Honestly, I think it would cost so much to bring the entire company up to speed, they may be better off just selling out to another airline that would put their equipment into place and just salvage the established routes and customer base. :x