Diablo is pretty much a roguelike with better graphics, albeit most modern roguelikes have much more variety when it comes to available races, classes, skills, monsters and other stuff. On the other hand Diablo made it easy for people who never before and probably after touched anything resembling an RPG. It appealed to the lowest common denominator, but in this case the lowest common denominator was a highly concentrated package of fun. Really, all you do is going down into dungeons, kill monsters, loot them, level up and repeat ad nauseam. Every fourth level or so the style of the dungeons changed, but the gameplay never really changed much.
But this curt description can’t convey just how atmospheric everything was, from the brilliant soundtrack to the gloomy art direction and the inspired monsters. Sure, you’ve seen them in thousand other games before, skeletons, demons, etc., but never before did they fit so well with the dungeons you descended into. It was a brilliant to-the-point action RPG that hasn’t been surpassed in style by any of its direct successors or derivatives.
This single player expansion from Sierra added two new dungeons, a new class and new stuff to loot. While not a necessity, it’s a nice addition to the original game and manages to be just as much fun. Of all the additions I really like the Hive dungeon, with its insect themed monster designs, which were quite cool.