I, too, have noticed that opening the driver's side door doesn't 'kill' (turn off) the car 100% of the time. I've been wondering what specifically determines whether is stays on or off... any thoughts???
Here's what I have found, and I tested it.
If you want: to get out of your car, but leave it 'running' then you just leave it in D (drive) and come to a stop. Then, open your door. Strange as this sounds, it will leave it in D, but apply the parking brake for you and allow you to exit the car with it sitting still (I have not tried this on a downhill, but it works on my driveway). It will beep at you, and let you press OK to acknowledge that the parking brake is on and the car is running.
If you put the car into P (park) and THEN open the door, the car will shut itself off. But, if you don't put it in P, then it stays on.
If you press the OFF button, while in D (drive), it will put itself into P (park).
I did find that all this "thinking for you" is making me fall into bad habits. I drove my wife's Chevy to a Circle K last week, and just blithely got out of the car and went into the store leaving the keys in it and running.