A few things come to mind (not meant to be exhaustive, I didn't want to repeat a bunch of things, though there will be some repeats)
- Solid global network with partners/Oneworld. AA has a less robust Europe/Asia network on their own metal, but they offset that quite well by having ideally located partners in LHR and TYO for connections
- They've been investing in clubs and it shows in places. The E club at DCA is great. The joint lounges at JFK are excellent. Work to do still on a number of existing clubs, but they are making effort
- Diversification of the LP earning system allows a level of flexibility in qualification, for those who need/want it.
- Warm, mixed nuts. This is immaterial, but I do just love those little ramekins on domestic meal flights.(UA has them, DL does not)