Three words: Customer Service & Culture
I have been a FF with Delta seriously over the past 10 years. I fly mostly international for work. I've been a Diamond every year since it was implemented. Not as happy with the revenue based way miles are distributed now but that seems to be prevalent in most FF programs or will be. I've had a lot of success upgrading to business on many international flights, I've never wasted a GUC, save them for those long flights to Sydney, Johannesburg and Dubai when there was a direct flight from ATL. I use miles for Y,B,M,S,H,Q,K classes.

When I fly with my wife, usually on domestic flights, we fly first class. I pay $ for my seat and use miles for hers. I heavily use the Delta AMEX Reserve card, the companion certificate gets used every year. I squeeze out all the benefits that come with a $60K spend. I like that Skymiles never expire.

Can't say I've ever had a bad interaction with a Delta representative in person or on the phone. Courtesy and professionalism are consistent and on point, sometimes it's surprising the way situations evolve and turn out. In hindsight, I'm left with the thought that these are my kind of people, overall they're very personable people, I appreciate that. Early on I tried to find that connection with Continental and then United, wasn't the same.

If I have a grievance, it lies with the Skyteam Alliance. The interoperability between Delta, KLM & Air France are more times than not, very seamless. Not true with the others though I've read Virgin is getting better.