Pretty much CharlieChapman took most of what I was going to say, especially the stereo part. However, there is one standard he didn't mention that I would love to see and that is the use of more than one button on a joystick. The one-button joystick/controller worked back in the days, but now it's limited and archaic. I know it's possible to use more than one button on an Atari 8 bit with a Sega Genesis controller, since I read it in this forum.
The one-button limitation sure don't work for shoot-em-up platform games. If Contra ever gets ported to the A8, I don't want to be forced to use Up on the D-pad/joystick to jump. I want it to be used, in conjunction with a fire button, to shoot up.