For me, it is the way that they treat their best customers. They just don't care. Every airline is having ontime performance problems. I have found, however, that DL and AA handle it much differently than US does. When the CP desk YELLS at you and says, you just called and we told you that we can't tell you the status of the incoming aircraft and you BETTER get to the airport NOW and then the flight cancels and they make you wait in line to tell you that you can only go standby in coach on the next flight as a CP who BOUGHT a FC ticket, enough is enough. It was bad enough they lost my luggage for a week in February and I didn't even leave the airport--on that flight, they so generously offered to refund my REFUNDABLE FULL Y FARE TICKET. I shouldn't have to demand that they put me in First on another carrier when I pay for a FC ticket. I shouldn't have to argue with the CP desk about the status of flights. I shouldn't have to worry about my bag not making it or waiting for an hour for my bag to come out. I shouldn't have to deal with dirty planes and terrible service in First on expensive tickets. I shouldn't have to deal with a crappy website that works only some times. So, I am not. I'll fly US when I have no choice because I live in PHL.
But, for those people in Tempe who say the customer won't pay for better service or that US's service is comparable to other carriers, well that is just ridiculously inaccurate.
For example, on a ticket that was less than what US would have charged me for a non-refundable guaranteed first, AA gave me the following on a refundable first class ticket for a transcon on a 2-class 763.
The seats were as good or better than what is in Envoy. There were 2 F/A's, the galley hag who also helped with drinks, and another F/A who would come around very frequently to help with the service. The plane was clean. Each seat had a large pillow and comforter in a bag. You board and they come around with champagne, water, and OJ. That is also when they hand out the individual IFE's. The IFE's have movies (watched Ferris Buehler and forgot how funny that movie is and forgot that Jennifer Gray pre-nose job was his sister), tv shows (I watched two episodes of the Office which cracks me up), music, and videos or you can watch what they showed in the main cabin on the screens above. The headsets are Bose noise canceling headsets and they are nice. Then they come around and hand out linens, warm nuts, menus and take drink orders. Then the hot towels. Then they take meal orders (we had shrimp appetizer, salad with choice of dressing, bread, beef tenderloin or ravioli). Then they serve cheese and crackers and fruit or ice cream sundae with choice of butterscotch, hot fudge, nuts, and whipped cream. Then they serve a baked on board chocolate chip cookie. Then before you land, they serve sparkling water with lemon and chocolate. A little different than US huh? Oh, and my Bose headset wasn't working properly and they said, no problem, we have extras. AA also has a magazine for "premium customers" that is pretty good. From what I hear, AA F/A's used to hate working first because it was so much work, but many are so sick of Ma and Pa Kettle in the back of the bus, that they would rather deal with their premium customers. So, contrary to what Tempe thinks, you can have the Ma and Pa's looking for a cheap fare and the premium customers looking to pay more for more.