To give you an idea of how innovative Diablo was in its day, let me outline the entire control scheme for you.
"Left mouse button."
Yes, that really is all you have to do (well, you actually also use the right mouse button to cast spells, but still, you get the idea). See a bad guy? Click on him to attack! See a treasure chest? Click on it to open it! See some money or equipment lying on the ground? Click on it to snag it! You get the idea. The simplicity is remarkable - the learning curve is about five seconds. That is the source of this game's awesomeness - it's so amazingly easy to put it in, log on to the network, play for a little while, pick up some goods and money, and leave. This simplicity is also what made the game so amazingly addictive - "All right, I'll only clear out ONE more level and then go to bed. I promise!"
Unfortunately, the simplicity also extends to other things. The game isn't very plot-heavy. Actually, the instruction manual sketches a rather good plot, but the game isn't big on developing it. Essentially, there's a haunted town, and there's 14 levels of monster-filled madness (or FUN!!!) underneath it. You can find books in the dungeons that will tell you some more plot, and towards the very end do the townspeople give you some "major revelations," but that's it. But honestly, no one plays Diablo for the plot. It is a dungeon hack to the core - like all those plotless D&D games where you went into a cave, kicked the crap out of some kobolds, then came out with treasure - and shouldn't be treated like a plot-intensive console RPG. The entire game is centered on killing stuff and breaking things using one of three character classes in order to earn EXP and money to buy better equipment that will allow you to kill more stuff and break more things. And to keep it interesting, there are plenty of different kinds of beasts, and most importantly: randomly generated dungeons. This is perhaps Diablo's best aspect - every time you start a new game, although the town layout remains the same, the game creates a whole new labyrinth underneath. The game allows you to keep your character out-of-game, so no matter if you beat it or not, you'll keep your EXP and items when you come back for the next round. And Blizzard's next amazing innovation (at the time) was its online FREE Battle.net network, allowing you to team up with your buddies and dudes to take on all the creatures.
The problem is - the dungeons, even with all the rearrangement, get boring after a while. There are only four tilesets, and eventually you'll learn all the different kinds of monsters. Once you kill the final boss and get all the best equipment, there's really no reason to play the game again. Blizzard realized this, and included an extra option in online play - you can play at a much higher difficulty level with all the beasts pumped up. The rewards are greater, but you'll never be sure of surviving the next turn. But even after a while of that, eventually you find the best equipment and kill the final boss, and that leaves absolutely no reason to come back. Secondly, Battle.net is known for frustrating bugs that tend to erase all your EXP and items just when you're getting up there. Thirdly, the character classes are too unbalanced; at higher levels, the Sorcerer is pretty much invincible. And finally, the Battle.net Diablo scene is plagued by cretinous bastards who don't respect the community and choose to murder their allies instead of the monsters.
The three amazing innovations of extreme simplicity, random map generators, and free unlimited online play make for a great game, but once the initial awe from all three wears off, it begins to get repetitive and eventually, you'll probably leave and never return. And if you haven't experienced it by now, you probably won't, as by releasing the (graphically, if not creatively) superior Diablo 2, Blizzard finished off the last of the original's appeal. That does not, however, bar the fact that in its day, Diablo was one of the best games around.