From what I've seen so far of the two Dark Forces games, they both seem to be impressively ambitious entries in a genre which was still very much evolving when the series began. The Sith Engine-powered Jedi Knight (Dark Forces II) is especially noticeable for its staggeringly large levels - both in terms of mission length and the sheer size of the rooms and other areas you explore.
As for the original Dark Forces, it distinguishes itself by taking the basic Doom formula and adding much more realistic and believable environments, concrete mission objectives clearly tied into the game's story as well as an interesting save system based around lives and checkpoints rather than the infinite number of reloads which most PC shooters rely on today.
However, as is evident from this video, there's a lot of good old fashioned Doom-style action in the game as well...
9