Instead of a joystick or D-pad, it has a flat circular dial for its directional control, referred to as a "control disc", which is also a button and is supposed to combine the functions of a paddle controller and a joystick.
It has at least 17 buttons, 12 of which as part of a number pad, akin to calculators and telephones.
The controller’s shape feels so awkward that it is downright uncomfortable to use.
The controller cannot be unplugged from the console without extensive modification, unlike most video game consoles, even during the second generation when the Intellivision was released, where there were alternate controllers you could use.
Mix all four of the points above, and you have controls that are simply bad for the console’s games. This is worsened by the fact that each of them has different controls.
Many of the games came with interchangeable overlays for the number pad, which are easily lost if you're not careful.
The interchangeable overlays for the number pad can reliably explain which button does what.
The action buttons are positioned on either side for lefties and righties.
The control disc is an early step in the transition towards modern D-pad.