The 10-minute turn was also helped by having customers who knew exactly what needed to happen. The plane parked at the gate, the engines were shut down and the doors opened, everyone was already in the aisle and got off right away, the incoming passengers were in line long before the plane arrived (as much as 30 minutes before, if I remember correctly), and the went right on.

A 10-minute turn is now impossible simply due to legal restrictions, which require the airline to know exactly who is aboard the plane. When the boarding pass was simply a colored card they could board with the rapidity of a Chicago or New York subway. While domestic boarding isn't quite as restricted as international, it still isn't what it used to be.