ijgordon

I would say that most of the complaints at DL revolve around the SkyMiles program (a/k/a SkyPesos or SkyDong). Many aspects are terrible. Miles have been devalued to the point where, unless you find partner award availability at a "standard rate" (don't ask what that is as there's no award chart), you're frequently not going to get much better than ~1.5 cents/mile of value and that's been going down. No free changes to award tickets except at the 75k+ elite level (AA offers free time/date changes to everyone), and no changes for anyone within 72 hours of departure (they will make exceptions, but not because availability opened on a better/cheaper routing). And mileage upgrades won't clear at the gate (but GUC/SWUs will), which is insult to injury because international J mileage upgrades require a high fare class (and a lot more miles than on AA).

Operationally and in terms of service people generally are happy. There have been a few major meltdowns in the past few years, but otherwise they're reliable. You find fewer "bad" crews - i.e., less variability. That said, their version of MCE requires you to "upgrade" into it. It can be capacity controlled and there are windows for 50k/25k level elites (72/24 hours), which isn't great (but exit row seats can be booked any time by 25k+ elites). And FC and their "flagship" DeltaOne J domestically have seen catering go downhill as they aggressively try to monetize first class sales (often selling cheap upgrades to non-elite; AA's LFBU doesn't come into play until all elites who want can be accommodated).

CoMooter

...and at least IMHO that would be UA leading the pack on that count! Died down a bit from previous years but during the reign of Jeffy things seemed to be particularly abundant...

3Cforme

Delta H, Q and K fares are upgradeable with miles on long-haul, and Delta miles can also be used to upgrade AF, KL, VS and AM flights.

There are plenty of 'I'm done with this airline' threads on the DL and UA forums. I find it interesting that, more than ten years after the DL/NW merger, some people are still whining endlessly - but continue buying DL tickets enough to be Plat & Diamond because they haven't found anything better.

ijgordon

K can easily be $900 more r/t over lowest available main cabin (e.g., NYC-PAR Nov/Dec), obviously more than AA's $700 r/t copay, but also you're out the cash whether you clear the upgrade or not. And I think it's way more than 25k miles each way. No thanks!
I think there are a lot of constraints, especially on AF/KL flights and VS may require high fares too. Probably somewhat better than what AAdvantage offers on IB/BA. I'm personally not that familiar with partner upgrades via SkyMiles, but the rules are kind of confusing (e.g., it says AF/KLM must be from Y/B/M fare but also says AF is "from Economy to Premium Economy to Business" which doesn't make sense.

nrr

It may be that switching out of your current system may be better, but in many cases one really has no choice: (1)home base city, (2)corporate contracts, (3)upgrade instruments which would be wasted if one switched, (4)rising to high level status could be difficult.
In my case ((2) doesn't apply) but (3) does, I have 5 SWU left, I (sort of) have to continue flying AA for awhile.

Tack

I've been EP for 3 years now, having switched away from AS/DL. I will re-qual for EP in November and I have been more than satisfied with AA. On-time, service, EP perks and award availability. I just flew back to LAX from MIA in paid F/C and the service was great. Food, wine and seat were as good if not better than anything I remember on AS or DL. In fact, I personally find DL to be coolly efficient and a bit full of themselves, I'm one that prefers UA to DL. There are always going to be service failures with any airline. It's expected. I book longer than normal connections and use the EP line if it looks like I might have an issue, and always receive great care. I get that YMMV, but to those that believe DL/UA and all others not coded AA are far superior? I just don't see it that way. Align expectations with reality and you'll be much happier. Cheers!