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.
Anticlimax Boss: After all you've went through getting there, the final boss turns out to be a barely animate blob. Considering the blob's former form was a contender for That One Boss, beating it up manages to be quite satisfying anyway.
The Dragon God is one of the most heavily hyped up Archdemons, appearing in the opening cinematic, killing you in the tutorial should you manage to defeat the Vanguard and generally being recognized in universe as a major power. Yet its actual "fight" consists of you running behind some pillars, activating a pair of magic ballista and then hitting it in the face while it's totally helpless to retaliate.
The skeletons of the Shrine of Storms just keep rolling around and breaking your guard. And don't get me started on those gold skeletons.
The Black Skeletons are the stuff of nightmares, even more so than their silver counterparts, as they are wickedly fast when they decide to start running, are insanely powerful with a sudden leaping attack that will destroy all but the strongest defenses if it lands, have high defense that will shrug off most damage, and will follow you farther than any other enemy you encounter. They're like Terminators with katanas.
The Giant Depraved Ones in the Valley of Defilement. They hit hard, have a lot of health, are not easily stunned, and worst of all, are in a level full of Bottomless Pits and poisonous water that removes any and all of your mobility (and none of theirs).
The Mind Flayers in the Tower of Latria are nightmares if not approached properly. At range, they launch very damaging rapid fire Soul Arrows. At close range, they use a sonic blast with a very weird delayed hitbox or fire a bolt of magic that paralyzes you, allowing them to pick you up with their tentacles and stab you through the head. Even if you survive being paralyzed and stabbed, they'll likely follow up with a sonic blast that will hit you before you even have a chance to stand up and evade. To top it off, all their spells deal heavy damage even when blocked and the paralysis spell is unblockable.
The Shadow Assassins summoned by the Reapers in Shrine of Storms 4-2. Ridiculously fast and hard to see, capable of turning invisible, absurd range and enough damage to instantly kill most players before they even have a chance to see what was attacking them.
It takes very little damage to make the Maneater flinch, but because it recovers so quickly, it's possible for it to recover from a counter hit and attack you before your melee swing animation is even complete. This makes slower two handed weapons, such as greatswords, all but impossible to use effectively.
The Maneater's pathfinding and targeting AI is spotty, to say the least, and its pounce attack covers so much distance that it will frequently jump off the bridge it's fought on and be forced to fly away and reorient itself. The combined result is the Maneater spending a full 30 or more seconds out of range for even the White Bow because it can't figure out how to land on the bridge. It's entirely possible for the second Maneater to arrive before you even take down a tenth of the Maneater's health, simply because it got stuck where you couldn't attack it.
The fight takes place on a rather narrow bridge where it is extremely easy to get knocked off by one of the Maneater's attacks or simply dodge rolling in the wrong direction.
The Thief Ring. While its effect doesn't sound too exciting on paper, it's actually incredibly effective, allowing you to sneak right past everything, sneak up on Black Phantoms and soulsuck them and become invisible to invaders unless they move in on you (auras and such are still visible though)... oh, and it works on bosses too, including the Final Boss.
Dual Katanas. While not gamebreaking for the singleplayer portion, these have earned a lot of hate in PvP. Their attacks are fast, hard-hitting, and stamina-economic. To top it all off, it can combo a main-hand push into an off-hand attack, back into a main-hand push. The added damage following a push combined with the aforementioned stamina economy makes it possible to kill full-health opponents with one combo. To add insult to injury, it is one of the easiest combos in the game to land, as you can chain into it with three different attacks, all of which are powerful tools on their own.
Almost everything in Demon's Souls's PvP could be considered a game breaker. By casting Light Weaponon an Iron Knuckle you can kill a high-vitality body form character with Second Chance active when they're at full health by headbutting them. It's an inescapable combo, unlike the Dual Katana combo mentioned above, which can be escaped. Despite all that, it's considered one of the worst set-ups in the game.
Obviously, having two experienced Blue Phantom players help you will allow you to storm through any level without you having to move a finger save for opening the fog doors.
In the very rare case that a friendly Black Phantom player invades, it can clear all enemies for you as they wont ever attack it. Of course, most of the time the friendly Black Phantom will be attacked by the host or a Blue Phantom as soon as it approaches them unless it took the time to send a message stating its intentions.
The Valley of Defilement has Giant Mosquitoes who are very annoying if you don't have ranged weapons or melee attacks that work well on them. There's also an annoying group of hard-to-hit rats in the first level of this section. The main mook enemies, the Depraved are also somewhat this; they aren't very dangerous singularly but they can come in hordes, like to sneak up on you, can cause status effects and have an irritating push attack (that can be outright dangerous in the first map with all its pitfalls).
Also, there are Phosphorescent Slugs who have spit and lunge attacks. They are slow to move and are among the easiest enemies in the game, but are quite dangerous when in large groups, which is how you will often find them in the Valley of Defilement. They are even deadlier when found in tight places coupled with moving lights that explode on contact.
The Storm Beasts flying around the Shrine of Storms, at least until you get the Stormruler, due to their tendency to fling massive homing spears at you whenever you turn your back on them, often from outside the range of most bows.
The background ambience and loud beating of the Old Monk's giant bio-mechanical heart in 3-2.
Memetic Badass: Due to his awesome looking Dark Silver Armor, ability to hold a giant mace in one hand and never let his shield down in the other, and fighting you all to protect Maiden Astrea, Garl Vinland was declared the manliest character in the game.
After a certain level. The Nexus, or starting hub, which seemed so safe, the only dangers coming from being an idiot (attacking some NPC's, falling long distances etc.), suddenly feels less safe when you notice corpses that weren't there before...
After the very first level you will only advance through dark hallways very slowly, with your shield raised high, and nervously rotating the camera around trying to spot enemies waiting to ambush you the second you move another inch.
Player Punch: Killing Garl tends to make people feel real bad about it.
The World Tendency system is certainly good on paper and well intentioned, but it unfairly punished new players by increasing the difficulty of the worlds while also weakening the difficulty for players who wanted more of a challenge. There are also sidequests and hidden weapons exclusive to either pure white/black tendency, and the tendency always adjust to the server average(usually grey tendency) when playing online after you turn back on the PS3, so many players felt they HAD to manipulate to system in order to get anywhere.
The Item burden mechanic only limited exploration and collecting, often at really inconvenient times. This forced players to drop certain items in order to get the new loot, permanently.
That One Achievement: Master Slasher's Trophy. Only two enemies in the entire game drop the Pure Bladestone ore, and the drop rate is infinitesimal. The only way to get it is to keep on killing them for hours on end and hope for the best.
Old King Allant. He's fast, has wide-sweeping, long-range, defense-penetrating attacks, and has high defense himself; characters that aren't built for melee may have great difficulty with this fight. Oh, and best of all: he has an attack that lowers your experience level.
Another boss that can be That One Boss is the Old Monk. It can be because if you play online, the boss can in fact be another player. Another player whose headdress shoots blasts that do a decent amount of damage. A skilled player being the Old Monk, especially if they have healing items and/or a weapon with a status ailment you weren't expecting, can be a real nightmare.
Most players agree that the undisputed That One Boss is the Maneater because not only is it in constant motion, has magic attacks that stun you and can easily knock you down the bridge you fight it in, but there's two of them.
Flamelurker, who is very strong, very fast and agile, has powerful attacks that are boosted by fire damage and often knock you down, becomes stronger and changes his attack pattern as he loses life, and until you beat him, you can't forge many of the weapons and shields that would actually make him a little easier to beat, since his soul is needed to forge demon soul fortified weapons, as well as more than half of the other special weapon types in the game.
The Valley of Defilement. It's dark, you have to traverse wide-open areas so it's easy to get disoriented, most of the place consists of swamp that will infect you with Poison and/or Plague, and worst of all, while immersed in the water, you're completely incapable of backstepping and rolling, and your movement is severely slowed, making you a sitting duck for enemies that can easily kill you in one hit (and are otherwise, unaffected by the water).
The damn bridge in Boletaria 1-2. It's not bad when you learn how to play the game more effectively, but good lord. You have the dragon shooting you with the fire, but the worst part was of course at the end. Archers in front of you but you can't go forward to much you'll provoke TWO heavy soldiers and you can't move back anymore because the damn dragon will blast you.
World 3, the Tower of Latria, which has monsters that can paralyze and then One-Hit Kill most players in a single attack. Plus, it's incredibly dark, and several paths drop off very suddenly, so you may be plummeting to your death before you fully realize you walked off the edge.
Oh, world 4-2. Dangerous shadow enemies that respawn until you kill the (also dangerous) Reaper that's summoning them, ninja shadow enemies that sneak up on and stealth attack unsuspecting players, Storm Beasts sniping you to death whenever you dare walk outside without a Thief's Ring, and 90% of the level is composed of narrow paths suspended over fatal drops. At least the boss is more fun than the silliness preceding him.
Maiden Astraea. An All-Loving Hero who came to the Valley of Defilement to ease the pain of the people that the world had literally thrown away. She eventually realized that "God" was actually The Old One (or at least concluded that no loving God would ever let a place this terrible exist) and, to gain greater power, took on a Demon's Soul. While plenty of other people became demons as well, Astraea's the only one to use her power to help others. Unfortunately, this was Not Quite The Right Thing, as the grateful residents went on to hunt down any newcomers in hopes of finding new souls to feed Astraea. Additionally, the souls she consumes all feed the Old One, meaning that her continued existence is actively putting the rest of the world in danger, thereby forcing the player to kill her to complete the game.
Yuria the Witch. Discriminated against and tormented since birth just because she's naturally gifted with magic. Her hatred of her oppressors drives her to become stronger, so she travels to Boletaria to harness the power of the Demon's Souls. Upon arriving she is defeated and captured by Executioner Miralda, then locked up, alone, in a dark tower, where she is tortured and implied to have been raped by the Fat Officials, repeatedly, in the name of "purification". Is it any wonder she so quickly takes to the Player Character, considering they may be the only person in her entire life to show her any kindness?
The Crestfallen Warrior. While he starts as a bit of a jerk to the player, if one keeps talking to him, he'll drop the attitude as he starts losing his memory and fading away, before outright yelling at the player to leave him alone because he can't remember who he or anyone else is. If the player exhausts his dialogue before killing the Tower Knight in 1-2, they will return to find him gone, and one can grab where he was sitting a Storied Soldier's Soul completely identical to the hundreds you will be munching on later on in the game. God. Damn.