I consistently think that I should stop being DL loyal and switch to a different carrier. AA is usually the front-runner since they have direct flights to where my parents live, I can fly BA direct to LHR, and soon I'll have a nonstop to HND. Every time I fly AA, however, I remember why I don't switch. The planes are always a mixed bag whereas DL has a pretty consistent hard product. The service is mediocre whereas I feel more at home on DL (could just be a familiarity thing). The 500-mile upgrade thing for AA Gold is wonky at best. I may not have had a lot of success with upgrades as FO but at least I didn't have to earn or buy certificates for it.
At the end of the day there are things that DL does well that AA doesn't. There are things that AA does well that DL may not. There are great employees and maybe not so great employees at both. I personally like the way DL treats me over AA, so I guess that's what makes them different.