I'm not an aircraft engineer, but I believe this is why certain aircraft (AA's 737s come to mind) have *incredibly* hard seat cushions in the exit row... because you might have to store the door there.

My guess is that some doors are heavier than others or have different width to height ratios, which change how easy it is for an average person to throw it out.

There's some interesting trivia you can learn from reading the safety card, paying attention to the safety demonstration, and generally being observant. For example, the FAA requires life rafts on airplanes (you'll usually see the ceiling compartments for them)... except it issues waivers to certain airlines for certain aircraft which do not fly more than 50 mi from land. If you fly American on an ex-US Airways plane, see if they mention life rafts in the safety demo.