Diablo doesn't really give you much of a story lead-in. You pick your character (choosing from 3 different types: Warrior, Rogue and Sorcerer), and then you're dropped off in the middle of a town, and start to wander about. As you do, you'll encounter a couple of people, who will explain some problems that they've been having. It seems that monsters have been appearing, and people have been disappearing. All of this stems from a cathedral on the outskirts of town. So, you head over there to check things out. This is where your adventure takes place.
Diablo is, essentially, a hack-and-slash game. This cathedral consists of many different floors, and, basically your entire goal is to make it to the bottom. Of course, as you make your way downward, monsters will get progressively harder. The town you started in, and the cathedral where your fighting takes place, is all there is to the game. Unlike the sequel, which had you traveling from one place to another, discovering new towns and problems, Diablo 1 has one place for you to go: Down. This isn't to say the game is without its exploration. Each level, you enter from a stairway from the floor above, and explore the floor to find the stairway down to the next floor. You may, however, find this stairway fairly quickly, without having a chance to explore much of the floor. While you can, if you want, just continue your trek downward, it's quite enjoyable to fully explore each floor, fighting monsters, gaining experience, and finding items.
As you explore the depths, you'll come across a variety of different items. Some you can equip on your character, and some won't be of much use to you which you can sell later on for a bit of gold. Since walking back up to the surface every time you needed a break would be a bit of a hassle, along the way you'll find scrolls which will let you return to the town, take care of some things there, and then return to where you had been. Between the items you find while exploring, and the items you can buy in the shops on the surface, you'll be able to improve your character's defense and attack.
Each character you can choose from has a different style of play: The Warrior's hand-to-hand combat, the Sorcerer's spellcasting, or the Rogue's distance attacks. You can customize your style of play by the weapons and items you choose to make use of, and how you use them. As you fight enemies, you'll gain experience, and when you gain levels, you'll be able to distribute points over your different skills. While there isn't nearly as much variety or customization as in the sequel, there's still a decent amount of flexibility.
The graphics in the game have held up well enough, with everything still looking fairly nice. The music provides a nice background as you make your way through the labyrinths, and the sound effects provide some nice touches. Overall, neither the graphics or the music and sound is anything that's going to wow anyone now, but still, they're nice enough, and they get the job done.
You can choose to play the game either in single-player or multiplayer. In single player, it's just you against the monsters, making your way down. In multiplayer, however, you can team up with a group of friends, or strangers, to combine your forces against the enemies. The online grouping system for Diablo 1 is still functional, so one can always log on to that to see who else is around and form a group. As well, one can choose to directly connect to someone else that you know, for play with a friend or group of friends. In multiplayer, while the main "goal" is still to complete the game, many people also get enjoyment out of the fact that you're able to fight other players as well. You don't have to if you don't want to (and, it's not my particular style of play), but, if you turn on the option to do so, you're able to engage in combat with the other people who you're playing with, for something to fight other than the monsters.
Related to this, there is a point to note, which is, in multiplayer, often there are many people who have chosen to cheat at the game, exploiting various bugs or programs to increase their character or give themselves items or advanced statistics or whatnot. I'm not here to tell you not to do that, although, again, I personally don't choose to. Still, it's something to keep in mind, and watch out for if it's not something that interests you. Personally, I feel that these things, especially in single-player mode, take much of the enjoyment out of the game, spending 5 seconds entering some numbers instead of a few hours exploring. Again though, that's your decision. If you choose to play without any of those things, just keep at it and you'll find a very enjoyable experience.
Again, Diablo 1 doesn't have nearly the same quantity of places and areas to explore as the sequel does. Still, there is still a great deal of nooks and crannies and out of the way places for you to find if you choose. For me, what keeps me coming back, is that it's not so expansive as to overwhelm you, but still varied and interesting enough where you'll be able to enjoy yourself exploring. As well, trying out different characters, and different items, can give you a completely new look on the game. While one can go through the game fairly quickly if one desires, I find it much more enjoyable to take it slow, explore, and enjoy myself.
Regardless of your style of play, Diablo has something for most everyone. While it does lack in some areas that its sequel improved on or added, the simplicity of Diablo 1 is still very nice, and there's enough variety to keep one coming back for more. Whether you adventure by yourself or with others, fighting against friends or enemies, whatever paths you decide to take, this is still a very enjoyable game. Worth trying even if you already have the sequel, and definitely worth checking out for an introduction to the series.