First of all, play this in a source port. I recommend Raze, which is easily the best looking and running port, though you'll have to download the .grp file for Twin Dragon from somewhere (freeware), since Raze doesn't support the loose files included in the GOG distribution. Playing Shadow Warrior in a source port is also an infinitely better experience than suffering through the buggy Redux version.
Explaining why Shadow Warrior is such a gem is somewhat difficult. In terms of gameplay, it's not much different from Duke Nukem 3D, with strong emphasis on key hunting, finding the level exit, and shooting baddies. Tonally, the shareware episode in particular is quite a bit darker, despite Lo Wang's one-liners easing things up a bit. Speaking of, recently there has been a debate about Lo Wang's lines and portrayal being "racist" and "politically incorrect," which they definitely are, but none of this is played straight, so I don't think this criticism really applies.
Rather, much of Shadow Warrior's appeal lies in its tight mechanics coupled with its relentless difficulty, which always keeps one engaged without ever feeling cheap. Coupled with Lee Jackson's highly memorable and atmospherically enriching score, Shadow Warrior is a weirdly addicting experience that simultaneously manages to never get boring or repetitive, despite the gameplay essentially not changing outside of the locale you're finding yourself in. Even though the expansions aren't quite on the same level as the original game, they include some neat level design ideas, and definitely are worth a playthrough if you enjoyed the main campaign. In general, you'll be hard pressed to find a level that doesn't do something cool or original in terms of design or looks in Shadow Warrior.
Overall, Shadow Warrior is another 3D Realms great, and well worth playing for anyone who enjoys classic FPS even today.