I played it quite a bit when I was 12, and gave it another run-through last year, so I guess I can give my review.
Ultimately, it was a case of Blizzard doing what it was good at in the '90s: taking existing genres and juicing them up into something that felt fresh and modern. That is, in fact, how Diablo felt in 1996. It was an isometric, realtime roguelike that dialed up the monster counts and the gacha factor, and there weren't already a million of those at the time.
These aspects still kinda work. In my last playthrough I played a Warrior and enjoyed muddling through without getting a decent weapon until Hell. That is, I had to improvise, make due with magic and a bow of maiming even though I wasn't a Sorcerer or a Rogue. I ended up with enough Magic to get the teleport spell, which I used to cheese Diablo's switch rooms. This is rather different from how I played it as a kid, and I wasn't expecting so much variability in what's workable as a build. I had always just used the New Game feature to reroll the dungeon until I found something that put me ahead of the difficulty curve, whenever things got a little too hard. You might even say that there's a skill curve, even though the gameplay is extremely stiff and it seems like there wouldn't be that much to it.
Other things it had going for it are multiplayer (in '96, you might have just gotten on dial-up, and this might have been your first multiplayer experience) and the aesthetics. In this regard, sequels could only ride the coattails of the original while at the same time making the text dumber and more cartoony.
That said, there's plenty of stupid bullshit, and I wouldn't seriously recommend the game when there are doubtless thousands that learned from its bad example:
- no run button; You just have to powerwalk everywhere (even to the shopkeepers, two of whom are preposterously far from town)
- the grid movement system is busted; You can never hit a moving target moving toward you at an angle
- even though aiming is (as a matter of positioning) skill-based, you also miss by diceroll
- sorc is overpowered, even if you don't exploit the knockback negation trick
- trying to weapon-swap like in Diablo II caused the game to soft-lock in my last run lmao
You cannot (IIRC) reroll Griswald's offerings, so if you're on dlvl 10 still using the club you found on dlvl 2, there isn't much you can do about it. It's kind of like a randomizer like that, which appeal to some but I reckon most people want the "vanilla" experience first— something tuned to be reasonable and obey some kind of logical progression. Including Nethack's "wish" mechanic would have been an interesting way to smooth out the RNG chaos somewhat, but that would have meant requiring the player to type something into a keyboard, whereas Diablo needed to run on a PSX and thus wasn't altogether a PC game. This, too, is typical of Blizzard.
Ranged enemies run from your melee attack, and they move at the same speed as you. This is excruciating to deal with as a typical Warrior, and particularly in Hell where half the enemies run from you, it makes Stone Curse way more valuable than any other spell.
Shrines (interactables in the dungeon that permanently change your stats) are deterministic, but you have to either memorize or look up what a "Benis Shrine" does before using it, or it might mess up your build a little bit. But that's just how the game is: it didn't really take much effort to build your character, and whether things go well or badly for you is on the knees of the gods anyway. That's how you should approach it, taking your chances and not being a pissbaby if things go badly.
Lastly, boss fights are awkwardly balanced: I got the Skeleton King quest but couldn't complete it until I found a book of Holy Bolt, because melee just as strong as magic. Diablo II "fixed" this by making bosses damage sponges, which is also retarded.
So there you go. There's kind of a fun game in there, if you're willing to look for it. These days you have to look pretty hard, though.