Even up to 2009, Diablo still keeps a fair share of people nailed and ramming their mousebuttons.
I won't compare graphics and sounds because this is a fossil of a game. (though the voice-acting is, as will remain to be in future Blizzard-projects, truly femonemal)
Diablo makes perfect use of a function is was unique for in its time: random generates levels. No new game is the same. To add, the quests don't even all show up in the first run, with the books explaining the main storyline of the Diablo Series (that then were to come).
Therefor, Diablo's replay value is near the roof, and that for someone writing at date of end septembre, 2009.
The three classes feel unique to themselves, if not just for their preferred weapon, but also looks, and you'll feel seduced to just play out all three classes, just to see how their three different armors look like.
In a nutshell, if you haven't played Diablo yet, and you have a chance, DO IT. This is a classic that explains to you why Blizzard is actually such a famed developer.