I am always leary of when oldtimers leave. Newcomers are more prone to trying stupid new ideas that kill value to the customer and bring short term revenue gains to the shareholders,.
But as far as the system goes,
-- (to the comment above) the published awards chart doesn't really exist in practice anymore. Mileage redemptions are all over the map, and for plain domestic coach tickets the redemptions are pretty steep already.
-- A simpler qualification system would be nice. Tracking mileage, legs, and dollars is annoying ($s I think makes the most sense but then again, why doesn't it include paid seats, upgrades, fees, etc that make up a good deal of AA's revenue) . Anyways, with simplification people will declare that some unfairness comes out of it --- No system is perfectly fair, perfectly calibrated, and the entire purpose is to drive revenue to AA.
-- Speaking of that last point, driving revenue to AA, I think there is a missed opportunity in that programs have zero features to entice people who fly with family. During the good times to travel, I buy 15-30 round trip tickets in a given year between me and the family and spend $8-15K in revenue to AA (6-10K of that completely discretionary personal spending) -- From that, I may qualify for gold based on the bit of extra business travel to push me over the top (Gold being relatively worthless) and the wife and kid usually come up short of anything. AAdvantage (and no program I know of for that matter) offers anything to help drive families to pay any premium to buy AA tickets for their trips. If I have other airlines at my local airport, I would have zero reason to stick with any of them vs just finding the lowest fares. Having some system where you can get some credit for spending on (a limited number of) family members when you fly together on the same PNR could bring value to AA.