I live 2 miles from American's headquarters. It's like they're not even trying to be cost effective. American is the anti-Southwest, whose headquarters are about 15 miles east. My brother is very conversant on the differences. I'll just quote the few I can remember from what he told me.
* Southwest taxis on one engine. Uses half the fuel. American taxis on two, by policy.
* Southwest flies one platform, Boeing 737. One set of parts, one set of manuals, one set of mechanic and pilot training, one simulator. American flies a few of everything. Multiple support and training lines.
* The backbone of American's domestic fleet is the least efficient aircraft still in use, MD-80. It can't be updated because there is no room between the fuselage and engine nacelle. It's also the noisiest, by FAR. Trust me, I live in the flightline. Southwest's 737 is among the MOST efficient.
* Southwest pays pilots by completed trips. American pays pilots for time from engine start to engine stop, leading to unnecessary delays.
* American maintains an adversarial relationship with employees. Southwest maintains a teamwork relationship.
* Southwest has seldom had a quarter where they lost money. American has seldom had a quarter where they MADE money.