Colourspace
Colourspace for the ST was a lot more complex than its 8-bit counterpart, and most significantly, far more controllable due to being able to use the mouse to direct pattern generation. It was also amazing to me at the time because one could generate graphics in super high resolution 320x200 in an awesome 16 colours out of 512! Yow!
The whole disk sleeve artwork is too big for my humble scanner, so I have just scanned in one of the cool parts with the alien dude who has a fish in his ear.