Dark Forces continues to stand it's own, and I still play it now and then (most recently a few months ago). I remember when I first played it, and it just sucked me in. The Star Wars atmopshere oozed from it in terms of the level design, enemy encounters and of course, the music. It instantly became my favourite game at the time, and I forgave it some minor flaws.
A good background story, that introduced a new believable character, and placed him right in the heart of events hinted at in the movies.
The levels were well-designed, with some cracking little puzzles. I liked the variety between the missions, with stark contrasts between some of the environments. Favourie levels include Gromas mines and the Ice station.
There were some pretty good secret areas dotted throughout.
The equipment you had, like field lights and infra-red, were just great.
The enemies were well-realised, and helped to give you a sense of place...and impending danger.
The tweaked movement controls, looking up and down, jumping, etc., were welcome additions.
I didn't like the absence of mid-level saving. I didn't really see much of a point in not having it.
A few levels were not as well designed, and did not really properly convey their locations adequately. Ramsees Hed was one...and coruscant was another.
The lack of NPCs...but at the time, this sort of thing was not in any other FPS either (that I played).
Jump puzzles. Well...I don't like them.