Most of these are very specific to my own situation, but here's why I fly Delta:

1. As noted by others, they have a pretty good clearance rate on domestic upgrades.

2. They are the only ones who fly actual planes (death to CRJ and such nonsense!) out of my home airport.

3. They are much cheaper on most of the routes I fly.

4. Minimum 500 MQM/DSM on every segment; for someone who has to take a guaranteed minimum four segments on every trip (home-hub-final destination and back) that works out to a minimum of 4,000 DSM (four segments x 500 miles x double miles for GM).

5. Contrary to a lot of the comments in the Delta threads, I find them to be extremely responsive to complaints/suggestions.

6. Insane and stupid loyalty to the ghost of NWA!