I’m currently testing out DL after having completed the Plat status challenge from UA. I’m happy to have status matched, as I can’t imagine jumping ship entirely. Few things I noticed.

1) DL staff in general are friendlier. This alone isn’t enough, but their positive attitude combined with front liners being more empowered to fix issues makes a noticeable difference in the passenger experience.
2) More investment in the customer experience. Sky Clubs are superior to United or Admiral Clubs. Safety videos are constantly updated. Highest in seat IFE avail. Only US3 airline to unilaterally extend status in 2021. Boxed/cold meals are better than the hot meals on competitors. More extras included for Y+.
3) The only remaining US3 airline w/o E/P/MQD requirements for international members.

But on an emphasis on loyalty, Delta has shortcomings
1) Smallest J class wide body footprint of US3, and toughest for upgrades. Only A330neo and A350 have the full Delta One suites too. UA has a 763 that has more J seats than any aircraft in the entire DL fleet.
2) Least likely of US3 to run a widebody aircraft on a domestic route.
3) Least generous upgrade policy for Y+
4) Tougher restrictions on use of upgrade instruments for others.
5) Cannot reach SkyMiles department directly.
6) Uncompetitive wait times for Medallion phone support.
7) Customer service on other avenues such as online messaging are more misses than hits.
8) Operationally, no longer the undisputed leader of US3. Post-COVID, faced a noticeable amount of non-weather related delays and even a cancellation vs. when flying UA.
9) UA has a superior SDC policy for certain elites.
10) As someone who prefers using tablet and phone for IFE due to Bluetooth for audio, DL is lacking.

If I wasn’t a hub captive or relied on loyalty for upgrades, I’d probably consider DL more if I actually bought tickets in premium. Otherwise, United seems to make significant strides in bridging the gap with Delta. A considerable amount of my flying is out of GUM/NRT(RIP)/HNL where the UA crews are surprisingly pleasant, so the service gap isn’t as noticeable vs. flying CONUS.