Anti-Climax Boss: Diablo himself is painfully easy to defeat. His only ranged attack is a simple blockable instant hit explosion but can be prevented by going melee, he's fairly susceptible to common magic, and he has pretty low armor class. Doesn't help that there is an easy way to lure him out without waking his mooks. Excusable because in actuality, the body you're fighting him in is that of a helpless child.
Arc Fatigue: Going through the four Cave levels can feel a bit barren due to its lack of quests. Black Mushroom and Anvil of Fury are the only quests, and due to how the random quest gimmick is set up, it's impossible to get them both at once during a playthrough, which means that you only get one quest throughout the entire Cave section.
Awesome Music: The first game featured some ridiculously atmospheric tracks.
Best Level Ever: The Hell Levels. The most atmospheric hell ever (if you prefer Gorn hell over fiery hell), and you fight the Devil at the end.
Big-Lipped Alligator Moment: For some odd reason, in the 16th floor chamber, there's a random Black Knight Mook amongst the much more powerful Blood Knight and Hell Mage enemies.
High level Sorcerer players tend to rely on just three spells. Mana Shield to protect against damage since the Sorcerer player's mana will be way higher compared to health, and Fireball and Chain Lighting to dish out loads of damage against enemies.
The vast majority of high level Warriors will rely on one-handed weapon-and-shield compared to two-handed weapons once it becomes clear that using a shield to have higher defenses and the ability to block attacks provides a lot better survivability against the stronger enemies of the Cave and Hell levels. It's also highly likely that Warriors will be utilizing a bow, or the Stone Curse or Fire Wall spells, to nail down ranged enemies who tend to retreat from the player. Especially to deal with Hell's Succubus and Mages.
The original game had the demon mages at the end of the game, who in addition to teleporting about, hit hard with powerful spells to make even ranged players cringe thinking about taking on groups of them (nevermind the melee players, to which those guys are absolute terrors). They came in swarms, had lightning attacks which could shred your HP down to critical in a few seconds, and teleported away from you before you could do any significant damage. The bane of warriors. It's not unknown to run out of money to spend on healing potions before you run out of enemies to kill.
If those weren't bad enough, they'd often be accompanied by Soul Burners. These winged demonic harlots would NEVER be encountered on their own, and while they weren't able to teleport around like the mages, approaching even an average-sized cluster of them with melee in mind still bordered on suicide for anyone not tooled-up to the eyeballs with special items and healing consumables. Succubi are a close second to the mages, particularly for warriors, as they also tend to come in packs and flood the screen with blood stars. They move at the exact same speed the player does, which makes catching up to them with melee in mind nearly suicidal.
Never mind the triple-immunes on the highest difficulties. These monsters are immune to ALL attacks save physical damage and Stone Curse. The only reliable way to kill them is to lure them out one-at-a-time, stone curse them, then smack them to death with either your own melee attacks or a golem.
The Black Death zombies are yellow zombies whose attacks can permanently reduce your HP upon hit. Rogues and sorcerers can simply pick them off at range, but they are a huge problem for warriors, whose primary attack mode is melee and are often forced to use magic or arrows to kill them without getting close. Thankfully, due to them being zombies, these undead monstrosities are slow and don't have as much of a chance of landing a blow as most any other monster that can be encountered in the late cathedral and early catacombs floors.
The Spitting Terrors. Their acid spits have a long range, high-fire rating and they leave a puddle of acid when they die. Warriors are guaranteed to be injured because they have to get close in melee range. Players also have a very high chance of meeting them: they can spawn in the catacombs, the caves and in Hell.
Ensemble Dark Horse: Griswold. Everyone who's played the first game loves Tristram's blacksmith to bits and it's one hell of a Player Punch when he turns up as a zombie in the sequel.
The Sorcerer was ridiculously easy to break by using the MS No-Stun bug. The "stun" that occurs if your character (or a monster) gets hit strongly enough is a huge part of the gameplay. But, Mana Shield can potentially cancel that. The problem is that it works by first applying damage to your life (as usual) but then converting it to your mana. If the damage is greater than your life, you're considered DEAD (and not stunned) for milliseconds until Mana Shield does its work. If your life ends up below your level (which is the stun threshold) you can never be stunned when using Mana Shield. Since sorcerers rely on Mana Shield in the normal gameplay anyway, well... To make it even worse, there's an otherwise feared enemy "Black Death" that when hitting you removes 1 of your maximum life, which you could use.
Expansion PackHellfire's easter egg bard was breakable without any bugs because of dual-wield. Just equip a haste/peril weapon combo together with an undead crown or helm or spirits for life-steal. Because peril does 1x WEAPON damage to the user, but 3x CHARACTER damage to the monsters, and such big life-steal will almost always get your HP back, such a combo can potentially cause total havoc.
The Blink bats. They're squishy, a single hit from most weapons is usually enough to down them, but as their name implies, they're literal bats that make good use of Teleport Spam.
Enemy Archers in any level, at least when using the Warrior. When playing as the Rogue or Sorcerer it isn't that big of a problem, as they have ranged solutions (and a case can be made for ranged spells, in spite of them using mana). But when playing as the Warrior, they tend to run from him quite a lot. And when massed, even when playing as the feeble Rogue and Sorcerer they can become a problem, as your character may fall victim of Death of a Thousand Cuts from all sides if they don't have enough armor.
The cloning glitch: if you picked up an item off the ground at the exact same time as left-clicking a potion from your belt, the item would appear in your inventory and the potion would be replaced with the same item. Extremely useful for fast cash or online item trades, as rare items sold well, but you could also have duplicate rings (the only item that you were allowed to wear 2 of the same thing). It could be easily abused to easy get a character to maxed stats as well, as the game included elixirs that each permanently increased a primary stat.
A possible item affix determines if an item increases or decreases the light radius (and thus how jay a player can see) around the player character. While the original intention of this stat is that a higher light radius is good and a lower light radius is bad (because stumbling around a darkened dungeon is not exactly the sanest thing to do) to the point that -lr items are considered cursed and a -lr modifier is considered the necessary drawback on several unique items to prevent them from being overpowered... This isn't exactly the case. Light radius is also responsible for determining just how far away monsters in the dungeon activate and begin hunting you down; a higher radius means that more mobs will be out for your blood and a lower radius means you can actually apply some stealth tactics.
In the Hell levels, Succubi's projectiles will never connect if they stand in a particular spot and you're just around a corner. You can then plow them with you own missiles or magical attacks at your own leisure.
It Was His Sled: After defeating Diablo, the Warrior/Rogue/Sorcerer tries to seal away the Prime Evil's Soulstone in their body. This game's hero becomes the Dark Wanderer in Diablo II that the sequel player character chases after. Like the Baldur's Gate series, this is largely because the sequels became more known, so many players started with them. But at least their identities are not revealed right away.
Jerkass Woobie: Wirt loses his legs and his mother, becoming a more selfish and sarcastic individual as a result.
Limited Animation: Doors, chests and other containers just go straight from closed to open or vice versa with no intermediate frames, likely due to technical limitations.
There's seven fire spells that can be added to the player's spellbook. However, players tend to just rely on three of them: Firebolt early on, and then switching to Fire Wall and Fireball at higher level. The Inferno and Flame Wave spells in particular are never used. Inferno is looked down upon because it's both rather weak, and needs to be at melee range to use, which puts you in immense risk of being attacked by melee attacks in return. As for Flame Wave, it suffers from an annoying bug where the "wave" is technically a wall of pieces, which the enemies end up avoiding taking damage because the gaps between the pieces of the Flame Wave don't deal any damage. Meanwhile, Guardian is just another version of Firebolt, and not only is its bolts quite weak by the time you get the spell, the Guardians tend to miss their bolts a lot compared to if the player just used their own fire spells. Finally, there's Elemental, which has some usage to kill hidden enemies around corners since the flame Elemental's gimmick is being able to turn towards enemies. But even then, it's viewed as overwork compared to just spamming with Fireball.
Blood Star is pretty underwhelming for an endgame spell that you can add to the spellbook. It's supposed to be the answer to use against enemies that are immune to Fire and Lightning due to Blood Star being a Magic-type attack that enemies are very rarely immune against, but the fact that the spell costs both mana and health to use along with the damage Blood Star deals not being all that high makes it too much of a hassle compared to just relying on the much easier and stronger Fire and Lightning spells along with just using basic weapon attacks. It's also been found to be much easier to just Stone Curse spam the enemies as an alternative Magic-type spell for said Fire and Lightning immune enemies. The reason being that while constantly freezing enemies with Stone Curse is very mana costly, that can easily be rectified by just stacking up on Mana Potions.
Moral Event Horizon: Archbishop Lazarus crosses this when he lures Tristram's citizens into the Cathedral to rescue Prince Albrecht, only to leave them all to die at the hands of the demonic Butcher. And then we find out that not only did this bastard kidnap Albrecht himself, but used him as the vessel for his master Diablo to walk the earth again. Diablo III reveals that he'd crossed this border long, long ago, being one of the major contributing factors in driving King Leoric to madness in the first place.
The ending cinematic, depending on which class you played as. When the player character drives Diablo's Soulstone into his/her forehead, the look of pain on the Warrior and Sorcerer's faces is authentic. The Rogue, by contrast, ends up looking like Barbra Streisand.
"The smell of death surrounds me!", the initial proclaim of the Warrior once he enters the Cathedral's first floor.
The Rogue's death sounds like less of a dying moment, and more like she just had an orgasm.
The player is only able to walk, which looks quite weird when you're in combat. Come across a spot where there's too many enemies to fight at once? Escape by slowly walking away.
Polished Port: Despite the Loads and Loads of Loading combined with taking up 10 blocks of a 15 block memory card, as well as downgraded sprites and character dialogue, the PlayStation port of Diablo works around its limitations to create an incredibly competent port that doesn't compromise on an ounce of the gameplay. It also packs per-pixel lighting like Diablo II would, adding more of a dynamic color to the game world without compromising its dark horror.
In the original release of the first game, every individual potion or item on your belt must be restocked manually after use. While freshly-looted items will fill the belt slots, more potions from the inventory will not automatically refill the spent slots. In the midst of a hectic run, this can get you killed if you exhausted all of your belt and have to spend time fiddling with your inventory to refill it with perfectly usable potions. The PlayStation port opted to auto-restock your belt with potions, and all future titles and releases would do the same, if not separate potions into their own dedicated button as a pooled resource solely for this purpose.
The Black Death zombies' ability to permanently reduce the player's Health stat by one point each time they land a successful strike is hated by a vast majority of players. It already feels bad enough to have Health stats permanently lost, but this also means that if you're hit enough times by Black Deaths, you could theoretically end up in a situation where you have just ONE point of total Health. It's such an odd mechanic that it's easy for newcomers to not even notice that they've permanently lost some of their Health points to a Black Death, and mods of Diablo I tend to just completely axe this mechanic from the game so that players don't have to deal with it anymore.
Sequel Displacement: The very first Diablo game effectively birthed an entire genre of action RPG games, setting into stone nearly every core design element you'd see even in the genre to this day. But it's so raw by today's standards that even lacking the quality-of-life and improvements of its own sequel makes it real rough for modern gamers to approach, the game is actually quite short overall instead of a grand, sweeping adventure, and said sequel was one of the biggest examples of an Even Better Sequel in the industry on its own terms. There's a reason why Diablo II got a complete remaster, while the original languishes in obscurity except among diehard fans and curious re-visits to see the genre origins.
Plaguewrath, a frustrating spitter boss. Basically he has a bunch of minions that all act the same way: they spit at nearly double, or even triple the speed of regular spitters, which means that often they'd drop a barrage of deadly spit that can drop you in seconds, before you can do anything. Add the fact that they don't have a melee attack, and unlike regular spitters, will actively run away from you if you try to get close, and you get one frustrating boss. Not to mention that the first time you'll encounter him, Plaguewrath himself takes forever to kill.
While it's a given that he becomes a huge overpowered Game-Breaker later on, the starting Dungeon levels as a Sorcerer can be a bit rough since a Sorcerer is barely able to handle a straight out melee fight, you start off with mana potions instead of health potions, and your early spell arsenal will be incredibly limited to the point that missing with any spell could be disastrous on your shallow mana pool.
The final Hell levels are this if you choose to play as a Warrior. Enjoy spending most of your walkthrough running around the levels chasing after Succubus that will run away, and constantly fire Blood Stars at ya. The same applies to the teleporting Hell mages in the final Hell level that constantly fires powerful Fireballs at the player, and then Teleports away once the Warrior gets close.
They Wasted a Perfectly Good Character: Due to her quests being cut from the game, Gillian the Barmaid ended up as a completely pointless NPC in the final product. There's absolutely no reason to talk to her except to hear a bit of her gossip about the townsfolk.
Thanks to longtime fans rebuilding Diablo I into mods over the years, it became quite clear that Blizzard left a ton of content on the cutting room floor. Some of the cut content include several quests (that even come with fully acted out dialogue) that never made it into the game that could have been interesting to play through. Some of these quests include clearing out a Tristram basement of snake creatures, or defeating Andariel out in the wilderness because she was leading villagers to their deaths.
The side-quests themselves suffer this as well. Once one pops up, all the characters in town can be asked about it for more information or occasionally confusion, but roughly half of them have proper conclusions; for the rest, you complete it, an item drops, and maybe your character makes a rare quote, and that's all that happens as no one will discuss it anymore. Combine this with the fact that the side quests are randomized per playthrough and some players may not even realize they completed certain quests.
The Scrappy: Wirt; he insults you all the time, he sales only one item at a time and forces you to pay for it just to look at it, then he charges you ridiculously huge amounts of gold to buy it. And he stays far outside of town while you can't run in this game.
Explanation 