Dark Forces was the first game to introduce a lot of new stuff. Like jumping, spiral staircases, secondary fire modes, you have to actually use some weapons mindfully in order to solve some puzzles, slippery floors, mission goals.
The interaction level in DF was never seen untill then, which combined with a rich SW ambiance made the game a very immersive experience.
I think the show that DF caused on most of us comes from the fact that we never played something like it before.
I hardly find as much pleasure on playing games now than I did back then, when every new game presented us a new experience.