Dual joysticks -- clumsy at first, then completely intuitive to the point where you wonder why other games don't use it (similar to mouse & keyboard for PC junkies). Excellent implementation of tech at the time, even if I did have to use some stupid rubber grommet to hold my controllers together when I played at home.
Speed -- It moves fast. You move fast. You shoot fast. Things happen... quickly. Lots of action games (HL2 in particular) like to break up your momentum by throwing things like puzzles and obstacles in the way, distracting you from a straight-out kill frenzy. Interestingly, since the Robotron hardware is so taxed by the game, playing it on an emulator actually makes it much faster.
Challenge -- This game hates you, but it doesn't feel cheap. When you die, you want to ram another quarter into the machine and take the stupid grin off those eight-pixel-tall robot bastards.
Transitions -- The "wee woo woo woo woop" screen between maps gives you a nice fast rush, then shoves you right back into the action to kill some more. Breathing room? Nein!
I am a firm believer that programming makes a good game -- tight controls (like the joysticks in Robotron) and responsive action (like your running about and shooting in eight -- eight -- directions) solidify the experience and make for a Zen-like experience, even after you've wasted all of your quarters.
Next week: Defender