The Original RPG Maker
The ability to create your own roleplaying game, with an intuitive toolset. Although the resources were limited, the package itself was so ahead of its time that the possibilities seemed limitless. "Rivers of Light" showed that anyone could make an immersive game with a meaningful storyline. "Lands of Adventuria" showed how you could maximize each and every resource to create stories in any genre. The ability to create and customize items, spells, and characters put the tools of imagination at your control. For many people, "Adventure Construction Set" was the first product that allowed people to explore the possibility of actually creating the kind of game they wanted to play.
It is hard to criticize the game. It maximized what you could get out of a Commodore while laboring under the obvious imitations. It was truly revolutionary.
The Bottom Line
If you have ever used RPG Maker in any of its iterations, here is the granddaddy of all those programs.