Two Delta experiences during the past 30 days:
International flight (Atlanta-Incheon and back) that featured broken seat trays and seats, slightly dirty aircraft, filthy bathrooms that weren't maintained for 14 hrs, indifferent/bored flight attendants who avoided work, marginal food. (Compare with Korean Airlines: outstanding service, superb food, immaculately maintained aircraft, constant attention and food/drink, spectactular flight attendants)
2) Skymiles award ticket redemption: hugely increased mileage requirements for domestic award tickets (from 25K miles to 37K or 50K miles for most flights), all kinds of creative and absurd ticketing fees ("partner airline" use fees, taxes that weren't previously charged, new additional fees for any flight that's earlier than 21 days). If you think you know the rules/assumptions of Delta's Skymiles program -- think again. It's become a kind of "bait and switch" scheme in which Delta unilaterally changes the rules and you're stuck
Review about: Delta Airlines Flight.