These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Play the Game, Skip the Story: Who was going to pay attention to it, anyways? If anything, Blizzard knew they shouldn't make anything too original.
Anyone who only played multi-player probably didn't even know there was a story; it skips all the cinematics and most of the background is in the manual.
Complacent Gaming Syndrome: Expanding on what was mentioned above - The game is played in three difficulty levels. When you beat one, you play the same five acts and their quests again. Very few who play on the public servers do anything besides pay other players in-game loot to beat all the bosses for them, skipping sidequests, so that they can sit and leech experience in games run by high-level characters/bots and shoot from level 1 to level 80+ within a few hours. This is justified when making a PvP character to avoid all the grinding, but most do it so they can rush bosses looking for perfect loot or just because they're bored of the repetitive nature of a three-difficulty playthrough.
Not to mention, when you play the game in multiplayer, you look up a stat-sheet on the internet, carbon-copy it in your character, and follow the exact skills in the playthrough. Blizzard was Genre Savvy enough to know that the same thing would happen in Diablo III and letting people customize stats would be redundant anyways, in part because of this trope.
Archbishop Lazarus, who played an integral role in King Leoric's descent into madness and eventual transformation into the Skeleton King, as well as manipulating him into torturing and executing many innocents in the guise of "protecting his kingdom" (up to and including his own queen), feeding Tristram's villagers and the odd adventurer to the demonic Butcher in the first game, and having Albrecht, one of Leoric's two sons, made a vessel for Diablo himself.
Maghda from Diablo III. In addition to her service to the evil Belial, which involves plenty of Human Sacrifice, she is very fond of Cold-Blooded Torture through magical and other means, and uses it for many of her sorcerous rituals, such as the illusion over the Black Canyon Bridge in Act II. She and her coven are directly responsible for the massacre of Alcarnus in Act II (including one hair-raising report from a refugee about people there being flayed alive) as well as the near-destruction of the town of Wortham, and the murder of Deckard Cain and the torture of the now-human Tyrael.
Zoltun Kulle. This is a man who attempted to usurp his Horadric brethren by using the Black Soulstone to control the Great Evils, even murdering and dismembering his own wife just to fuel his twisted experiments with the Soulstone. Is it any wonder the Horadrim went to such extremes to keep him down?
Adria from III. She pledged herself to Diablo's service since first meeting Aidan after he became the Dark Wanderer, spending the time between Diablo I and III gathering up the power of the other Lords of Hell so she could use the Black Soulstone to bring about Diablo's rebirth in the body of her own daughter, an act that in the setting would damn Leah's soul to hell.
Every single atrocious act in the history of the series forms a blood-stained breadcrumb trail straight back to the Prime Evils themselves, with special mention going to the titular Lord of Terror himself, whose diabolical influence led many of the ones mentioned above into crossing their own Moral Even Horizons into complete monsterdom themselves. Of course, what do you expect from an entity made from pure condensed evil?
Demonic Spiders: Numerous examples. Exactly which monsters qualify varies depending on your character build, but Oblivion Knights, Gloams and Stygian Dolls from the second game are some of the worst.
From the first game, there were the demon mages, 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) 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.
Hammerdins (A Paladin that works off spamming blessed hammers) unlike other spellcasters, don't hit a brick wall with immunities, and needs no tactics but "spam blessed hammer" and is pretty much the only build good for both PVP and PVM.
Genius Bonus: Balrogs being called Megademons in the second game. That's more or less a direct translation (from Sindarin) of "Balrog".
Goddamn Bats: Plenty of them, but the Leapers most definitely qualify. They attack by jumping, and are knocked away every time you hit them. They are also fast enough to dodge ranged attacks, and always attack in large numbers. The pigmies in act 3 swarm you in large numbers or attack with their blowpipes from a long distance, and their shamans can revive them. Their undead versions are even stronger and explode when killed. Then there are the imps from act 5. They teleport, shoot fire from a long distance, and mount towers or war beasts where they're even harder to touch. And finally, the suicide bombers. They charge you at high speed and explode, knocking you back taking out a large chunk of your life and possibly freezing you.
The druid can summon these to his side in the form of ravens. They barely do any damage, but they are fast, cannot be attacked in any way, and can blind anything they attack.
The mummies, who spew clouds of poison in death and can be produced by a Mook Maker, the saber cats and slingers, who come in swarms, can throw exploding or poison potions, and move really fast, but die quickly, and anything that can poison you in Hell.
That One Boss - Formerly (and for some builds, still) Duriel due to some loading issues early in the games life that resulted in him killing you before the game loaded his lair. Even now you spawn right on top of him, and if you get close to him, you are slowed heavily (making melee next to impossible without prepping thawing potions, something otherwise unused). The arena is also the smallest of all the bosses, making Hit-and-Run Tactics virtually impossible.
Nihlathak (thankfully optional, unthankfully Randomly Drops the item needed to access the 1.11 patchs added bonus boss) thanks to his corpse explosion (already considered the best necromancer skill in the hands of players, it deals a percentage of a monsters hitpoints in damage by blowing up its corpse. Only a handful of obscure items can prevent it.) abilities.
The Lord de Seis is a mini-boss summoned before you fight Diablo. He is famous for his herd of ultra-powerful Boss In Mook Clothing minions and the aura he grants them flattening even powerful players.
The Scrappy: Wirt in the first game. He insults you all the time, he forces you to pay just to look at his single item, then he charges you ridiculously huge amounts of gold to buy it and he stays far outside of town.
Take That, Scrappy!: Blizzard acknowledge his scrappiness. You find his dead body in the sequel and when clicked, a fountain of gold pops out.
Amusingly, some players were confused about a rainbow in a screenshot of Diablo III, even though there was a waterfall as well, so that rainbow was going to be there anyway. Reality Is Unrealistic.
The Woobie: Marius. Unlike the protagonists, he's just a poor shmuck with no power dragged into the whole mess, scared out of his mind by all the death and destruction happening around him. The only time he tries to do something, he actually makes things worse. In the end, he dies in an asylum, at the hand of Baal, begging for his pathetic life.
Woobie Species: Ultimately humanity, formerly known as the Nephalem. Offspring of renegade angels and demons, the Nephalem were more powerful that either angels or demons combined. Unfortunately it was this reason, as well as the retribution that would come if the High Heavens and Burning Hells discovered this blasphemous co-mingling, that prompted the renegades (including the top angel Inarius, who would have been the father of the Nephalem) to call for their extinction. When the demon Lilith came to their aid and slaughtered the renegades, Inarius took his former lover and banished her to the void. Then he attuned the Worldstone (the main setting of the series) and proceeded to leech away his progeny's power until they became weaker and weaker through each generation, eventually becoming mortal humans. And of course it doesn't end there. When Lilith finally makes it back from the void, she tricks a human by framing him for the murder of two emissaries working for the Cathedral of Light and the Triune (both of whom were secretly led by the Prime Evils and Inarius to lure humanity to their side) and then, while disguised encouraged him to gather an army of Nephalem to fight the injustice of their oppression. When the ruse is revealed, she sends her human pawn near the Despair Event Horizon, and tries to use the nephalem army to crush all opposition. If you take this in another perspective, Lilith, the lone demon responsible for saving the nephalem from extinction, is now using them as puppets for the sake of gaining power. And to put even more blocks of salt into the wound, when the angels discovered Nephalem and their world, they made a vote on whether or not to eliminate all mankind. Only one angel saved them from extinction, and he would be responsible for giving power back to the human heroes leading to them ending the Great Conflict and saving Heaven and the mortal realm from destruction.