Domestic connections in the US are problematic because there's always bad weather somewhere in the country that filters through to the major hubs causing delays and cancellations. I personally avoid American carriers wherever possible because their in-flight service is bad. But I'm not entirely sure the problems with domestic connections are entirely their fault.
One thing that drives me insane about flying on a plane full of mainlanders - if you have a seat in the middle bulkhead, everyone will attempt to cut across the plane in front of you to go chat to their friends in the tour group sat on the other side. I have experienced someone trample my foot while I was sleeping multiple times. Now I just put my leg up across the way to block and if they try to pass I say NO.