They basically invented a genre with this game. They took rogue elements, combined it with a gauntlet type of gameplay, added a few drops of rpg and the hack'n'slash genre was born.
The game was also much deeper than most people realize. Have you ever experienced with light radius ? Have you played a combat sorcerer ? Did you defeat Diablo in Hell mode ? Did you ever reach level 50 ?

Stop comparing it to RPGs, compare it to action games. It has much more depth than anything that played as smooth as this, when released. It's a fun action game where it's easy to play with friends but still has enough depth to make it highly replayable (and with random dungeon generation).

Real time combat wasn't a huge thing back then, outside of games like Eye of the Beholder, which had semi-turn based combat.

It spawned a whole genre (arpg) and was one of the first roguelikes to make it big on a big stage.

Whether or not it stands the test of time, depends on your personal preferences, but since I never played through it back in the day, I'm loving it.

Not really.

Rather, this was the descendent of games like ROGUE. In fact, the original idea for Diablo was that it would not be "real time" but rather would be "turn-based." EXACTLY like "Rogue."

My first experience with a "Rogue-like" game was actually on the old TI-99/4A, a game called "Tunnels of Doom," which was entirely new to me at the time, but was, in fact, just a graphically-improved "Rogue." It allowed a party of up to 4x players... four different classes... random level generation... everything you get in Diablo, but Diablo did it so much more immersively.

I like Diablo (1) better than the later games because it's so much SLOWER. More thoughtful, less "rush in blindly" friendly. You aren't faster than your enemies, in all cases... in fact, not in MOST of them. You have to be smart... to think, to plan, to prepare.

By contrast, I'm currently (after playing through Diablo 1 twice in the past few days) going through Diablo 2. It's a far more POLISHED game, but things move too fast, and you're far too powerful relative to the enemies you face (countered somewhat by facing an awful lot of them at once, I suppose!) Especially once you get a companion to go with you... my Rogue companion is very high level and basically, between the two of us, we can clear any level with arrows or bolts in a few moments. So, I spend more time slogging back and forth between setpieces and where I can sell all my captured junk than I do actually fighting. And yes, that includes my perpetual stock of 20-ish scrolls in my Tome of Town Portal.

Simple stated, Diablo (1) is a totally different... and to me, more satisfying... game than Diablo 2. As for Diablo 3... I haven't even played it, but from what I've seen, it doesn't interest me very much.

For anyone curious about what, to me, preceded Diablo... check out this video of "Tunnels of Doom."

https://youtu.be/1LlUCZs1KZA

Two words: Online Multiplayer.

Back then, online multiplayer was a brand new concept, and Diablo was one of the first games of its kind to offer online multiplayer. These days we take this sort of thing for granted, but back then it really was revolutionary.

Elder Scrolls didn't have that. Few games did, really.

For me back in 1996 was the very Dark and Gothic ambience. I remember playing the "Spawn" version along with WarCraft2 as demos in my Uncle PC, the demo disc came with some PC Games maganize, i can't recall which one... i played the demo and it was super spooky, the music, the monsters and the difficulty we just got super hooked to this game. I remember the very first time i saw the Butcher, my uncle was playing the demo when he open the door and the Butcher came straight to him and wack him in one hit... that was one of the most epic times in PC Gaming for me. I then got my own PC and got myself a copy of the full game, i played this this since 1997 all the way to Diablo2... even though i liked diablo2, it feel underwhelming for me. The Gothic ambience was toned down, the demons were less scary and more alienish, and the soundtrack was not on par to Diablo1, that's just how i feel about diablo2. So i finished Diablo2 and and went back to play Diablo1 until Vampire the Masquerade - Redemption came out.

I'm starting to see why so many prefer Diablo 1 over 2, now that I'm actually playing through it, myself.

I played Diablo1 to it's knees, or maybe better said my knees. The feeling was immersive and the crazy random dungeons made you never sleep. Then Diablo2 cam and of course as people said, it was a better game, but the feeling of it's first touch was left outside. I will buy Diablo1 again and try it as I like old games.

This is NOT a RPG in any sense, if you want an early real RPG, we have Eye of the Beholder and the similar series. They were challenging, no hand-holding whatsoever.

Right now I'm playing Colonization, in my own opinion, one of the best games I played before, but not RPG.

Then Planescape: Torment! Now we talk RPG, Deus Ex was absolutely great but still not a "true" RPG.

But each to their own, I don't really like to put labels on games, people experience them differently. but the above cases I mentioned shouldn't burn me for heresy?

To me, it was all of the following:

Gloomy, dark atmosphere.
The monsters were convincing and well-made, the environments had a great atmosphere to them and the ambient was creepy.

Challenging but fun game-play.
When you died, it was always your own fault for not being careful.
Of course, there were exceptions, which was the deaths caused by the CD-ROM drive windup delay when walking into a group of acid-spitters (the game kept on going in the background, and when it finished loading the game would play at super-high speed to catch up).

High replay-value thanks to the randomly generated dungeons and loot.
No two games were the same, and you would always have a fresh experience. Having multiple difficulty levels also helped to keep its challenge up and allowed you to use your character you had been attached to.

Online Multiplayer
I played this a LOT online, much to chagrin of my parents. I was a member of a clan too.
It also introduced me to online multiplayer and the community built around the game (I used to chat a lot too).

I really loved this game when I was young. Now, after not playing it for 15 years or maybe 20, I got bored after 2-3 runs. Multiplayer? Omg, there are ton of much better newer games now for playing with other people. But still, Diablo 1 has some amazing feeling of quality and balance, such a solid masterpiece, it's like a memory how amazing was Blizzard before. Though too small for it's price. So it's just rip-off of old fans for nostalgia.. but who cares, better this then nothing imo.

It was the first of the whole action oriented "rogue" fantasy games. nothing iike it had been done before.
yeah, it seems limited in options compared to It sucessors, but in 1996 nothing close to had been seen before And it set a new standard for on line playl

I played all the games you mentioned at the time, RPGs were and continue to be my thing. I don't know how to explain it to you... Diablo was a game that invented a new genre - the action RPG.

If you really want to understand what Diablo was and meant at the time, I suggest you read "Stay Awhile and Listen" by David Croddock. It's a detailed account of how the game was made and why it was unique at the time.

https://www.amazon.com/Stay-Awhile-Listen-Legendary-Video-Game-ebook/dp/B00G8UL474

Well let me ask you this. Would an Mario Kart actually bring new Fans to Gran Turismo or Grid, because that's the Problem here. Diablo and Diablo-esque Games are in the ARPG Genre and Hack'n'Slay Genre for a "Reason", and your comparison would be like saying that Dark Souls (which is also an Action RPG) would bring new Fans to Divinity Original Sin 2 or any other proper cRPG.

The core and essence of the Hack'n'Slay's were never drained from RPGs, but another Genre / Subgenre -> Dungeon Crawler / Roguelike but mixed a bit of RPG-Elements into it. Based on that the begin with Diablo, but wanted to be more accessable and more Actionbased, plus add Multiplayer Components to it, so the closest comparison which you truly could make, would most likely Gauntlet and not any "Major RPG" which you try to force upon, and even your "Diablo 2" example which you try let stand out a bit, will never be comparable to the complexity of proper cRPG, but that's what it never is meant to begin with, but rather to have an Fun, Actionpaced Game with some RPG-Elements and Multiplayer Emphasis. The closest Hack'n'Slay / ARPG to an RPG might be possible Sacred and / or the classic Divinity Games(though from the Pacing of this Games i would consider Divinity already as proper RPG's instead of ARPG / Hack'n'Slay).

But to give my Answer why i was hyped about Diablo:
It was easy accessable but still with enough complexity and indepth if you want play longer session, fun Multiplayer and an Unique / Amazing World / Lore / Dark & Gritty Atmosphere which kept up for me(yes i know unpopular opinion) even with Diablo 3. I enjoy and love Grim Dawn, on the same Level as Diablo, but for me it never can replace Diablo simply for the lore / world reason and even in Diablo 3 there are too many aspects which cry "that's diablo" might it be characters, enemies or what ever.

Diablo was the first game where I felt like a part of an online community. Virtually every game before Diablo and many after it used crap like Gamespy or Won. The multiplayer aspect was bolted on and typically wasn't the feature of the game.

Diablo allowed you to click right into multiplayer. You're in a chat room and can click into someone's game immediately. You could leave a game and go directly back to chat and join or create another game. That was huge back then.

All that is needed.
The Warrior, The Rogue, The Mage. The Trinty of every RPG out there. You assume that you'd have access to those games..
Where I grew up, I never heard of Arena, nor Daggerfall. I learned of Might & Magic through Heroes of Might & Magic 2 but I never once saw Might & Magic series in any store. My only source of computer RPG's was Diablo, WarCraft I & II, and the duo of Heretic and HeXen. Yeah, that's right, I just called those two games for RPG's, because what I really mean is Fantasy games, which is essentially what Diablo is.

Point is, unless you lived in a fairly active and lively major city, you'd be lucky if you could get Mega Man in a video game store.

But assuming then yuo had options for all these games and the money to buy them; Why get Diablo?
Well, because the box art intrigues you, and hte word of mouth attracted you to it, and when you saw it with your own eyes, how scary, oppressive and creepy the game was and the amazing Tristram Muisc, you couldn't help but to admire the game and get sucked into it.

That's why Diablo 1 was so successful, as well as my next point; And look what you lost with Diablo 2. Here's my second point. You ask about the three characters, the three classes: But that's the thing: You can be a Magic user as a Warrior if you want. You can be a Paladin. You can be an Arcane Archer, using bows and magic spells, or you can be a Magic Knight as a Sorcerer and so forth. You had options in Diablo 1, while in Diablo 2, you are forced to be one thing - and nothing more.

Likewise the feel of Diablo 2 is completely different. It doesn't feel isolated, it doesn't feel oppressive anymore. It's just not as good as Diablo 1 was. But that's a subjective matter of taste.

Diablo was later ported and released for PS1 which is a fairly intersting version indeed. Featuring some improved visual fidelity in some aspects and reduced in others, as well as completely dubbed in 3 other languages ( Swedish, German, French ), where in Swedish dubbing you can hear suhc famous Swedish actors like Claes Malmberg recite the role of Griswald the Blacksmith, and laugh how awful it sounds. The most amazing part of this port though is it having 2 player - same screen multiplayer - as well as the complete backstory of Diablo narrated by the same amazing voice actor who narrates everything in Diablo 1.