Insanity. Millipede is distilled insanity in a video game. Compared to it, Centipede feels like a child's game. There are so many things happening at once on the screen that it is literally impossible to keep track of it all. The best you can do is to keep moving and keep shooting and hope you last long enough to earn another life. It is absolutely absurd that a game this frenetic can also be this incredibly fun.
Another innovation that sets Millipede apart is the intermission waves. After successfully destroying a millipede you are regularly treated to a swarm of some other kind of pest. This swarm rains down upon you and can be shot for mega bonus points, but you have to be careful because they come at you quickly and can easily overwhelm. With the right swarm of pests you can rack up major points, clear the screen of mushrooms and even score an extra life.
Millipede is pure action. You will not find a vertical shooter for the Atari 2600 with faster-paced, frantic action. I was initially intimidated by Millipede's blazing game speed and level of challenge, but after a few games I settled right in and now it is a must play for me anytime I'm looking for some shootin' action!