Diablo's soundtrack is an amazing thing - although there are only 6 tracks (and I do consider that a downside) they capture well the entire feeling of the game. The intro is dark and stirring, of course, but it is the theme of Tristram that really gives one a sense of the feeling. A town, beset by demons and the undead, with many of its citizens dead or insane. The twang of the strings suggests a bucolic joy, twinged with a darkness or sense of loss. Of the four level types, I enjoy dungeon the most - it has a delicious sinister feel. Catacombs feels dark in its own way, but less enjoyable and more repetative. Caves both pleases and annoys me in ways the other tracks don't - its random and strange, with psycho sounding guitar riffs, but some of its strangeness is just silly - there is one part in particular that sounds like someone running a vaccum cleaner while in the background the music from Jabba the Hut's pleasure barge. Hell is militaristic, insectual, and cultish, but I am not sure the music (particularly the baby cry/whine) matched the area all the time. The spookier parts, where sound is warped, appeal more so.
Overall, an excellent collection of music, if only it were longer and more varied, but still as dark.