Diablo is an amazing game and definitely a classic.
One thing I'll say is that if I'd played it "back in the day", I would probably be giving it a 10. I can imagine the impact that this game had on video games back in the day. And even playing it in this day and age, the gameplay still holds up well. Playing the game really made me see how much Diablo 2 improved on the first one.
The gameplay is pretty similar to D2 - you'll be doing alot of dungeon crawling and killing alot of enemies, levelling up and finding new armor and weapons along the way. The game basically has you starting in a town, and there is a church that you must enter and descend its floors to the very bottom. Along the way, you'll discover many things and fight alot of interesting enemies. The fact that you spend the entire game simply descending the floors of a single tower isn't quite as interesting as travelling across an entire world like in D2, but the floors are varied enough so that there is a variety in the enemies that you are fighting.
The one flaw that just doesn't hold up by todays standards is the fact that you can't run. This doesn't seem like a problem at first glance, but when you've got 10 enemies attacking you with ranged attacks, and you try to chase one enemy down to attack it, but that enemy walks away from you at the EXACT same speed that you walk (i.e. YOU CAN'T CATCH UP TO IT!), WHILE you are under attack from 9 other ranged enemies... it can be pretty annoying.
In the end, Diablo is a really fun game. It took me 11 hours to play through the entire game, and there are 3 classes to play as, plus an online component, so you'll definiteyl get bang for your buck (considering you can find the game for about 5-10 dollars on ebay these days). While the game has shown some age over the years, there is no denying that Diablo is a classic and that the gameplay still holds up fairly well even 12 years after originally released.