Good, fast flow to the game with lots of ball movement (see: ToM)
'Mystery' targets that give a random award when they are hit re: SS, Cyclone and so many others. Of course this ties into the code aspect as well; for that matter, all elements of the game that are random (the game should be a bit different each time it is played). See: WPT and the random cards the player is given when they hit the ramps, there is no way to predict it
Sub-playfields give the player a more rounded experience, it is like getting 2-3 pinball machines wrapped up into one
Entertaining video modes (Williams IJ), hopefully these will develop nicely with LCD's!
Strong sound effects- this has been a lost 'art' since the days of '70s and '80s pins