United holds connections smartly, as we all know. It's all about doing the math and figuring out the tradeoff between the passengers making their flight and the potential knock-on effects of a slight delay. The point is, AA does not even try to do this calculation, they just set a rigid D0 policy and stick to it. United has invested the money into the tech to do those calculations automatically-- all their customer-facing tech is better than AA's, so it wouldn't surprise me that they're ahead on backend stuff too.

I have been flying a lot more United recently since losing AA status. They haven't been amazing but these little things do tend to make me more likely to keep going back to United.