Diablo 1 is a classic game that was made by the old Blizzard (that created quality games and franchises), and this game was the start of not only the Diablo franchise, but an entire genre, the point-and-click, hack-and-slash game (a type of action-RPG). Part of the appeal is the gameplay, where all you really need to do is click, click, click, click to kill the bad guys! So, it is really, really easy to pick up. What keeps you playing can vary player to player, you could get sucked into the immersive atmosphere and world of Diablo, you could be looking to have the best gear so need to grind out just one more dungeon run, you could have loved the free online play...whether that have been grouping with a party or PK'ing (player killing). Whatever it happens to be, the likelihood is that something will hook you in and make you love this game.
That being said, Diablo isn't a perfect game. There are 3 classes to play, and they do feel a bit different from each other (although maybe not as distinct and later games), but they also are not very balanced. It is well known that an end-game Sorcerer is the most powerful class in the game, with his ability to still wear the best armor in the game, like the warrior, but also be able to have a deep mana pool and learn the highest level of spells, which allow him to decimate most enemies, rooms, or even floors, without much effort. The general lack of mobility for characters (other than a phasing sorcerer) is also pretty painful. The lack of a larger inventory, or more buttons for skills / spells is annoying, but the argument could be made that that lends to the strategy / management of the game. Item drops can be a pain to locate and get lost among the tiling or corpses. In multiplayer, hacking and getting player-killed (when you weren't looking for that) was rampant and could destroy the fun of online play.
I'm giving Diablo a 10 because, at the time, it was a 10. I played it when it was released and there really wasn't anything like it. The fun you could have on battle.net (multiplayer), and just the fact that every dungeon run was different due to the randomly generated layouts really made the game enjoyable and highly replayable. That being said, the game hasn't aged well, but it isn't unplayable (it still brings heavy nostalgia for fans, and has a great atmosphere), but there are just way better games out there now...including the other games in the series. If you are still curious about how the genre and franchise started, Diablo is worth checking out and playing! However, if you are looking for what may currently be the best in the genre, you'll have to look for something else.