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.
YMMV: Demon's Souls
Accidental Innuendo: Sometimes, the Maiden's line "Touch the demon inside me" sounds a lot different from what it should mean.
Not to mention the item you use to give your weapon magic damage, Sticky White Stuff.
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.
When you think about it, it makes sense. What King Allant has been reduced to (a pile of sludge that could barely harm a bug) shows how relying on the power of Demon's Souls will ultimately destroy you.
Or rather, King Allant was regressing, since the player has cut off his formerly steady supply of souls, and the Old One was now sucking the souls out of him for nourishment.
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).
Game Breaker: The Firestorm spell, which unleashes huge flames all around the user. Invading Black Phantoms love to spam this.
There's also the long spear, heavy armor, heavy shield combo. With the Brushwood Armor set and a sufficiently built up character, and a sufficiently strong spear, you have a powerful equipment based character that, while slow, can shrug off most hits, has a shield that can significantly reduce magic damage, and can attack without lowering his shield. This is effective against many of the enemies (and several of the bosses) of the game. Just level it up and add a bow for long range enemies, and the Keel Smasher for the more massive bosses, and you have Mighty Glacier that will shrug off all but the mightiest attacks, and can deal death in all 32 flavors. You can work on magic and healing spells in your second playthrough.
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 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.
While not necessarily a game breaker, the Anti-Magic Field miracle can transform world 3 from "That One Level" into "Happy Squid Head Stab Time", since the squid headed guards (who have a powerful distance attack spell and a paralyzing field spell that they use to render you defenseless against a nearly 100% fatal attack) rely entirely on magic, and once you're up close to them, all they can do is try to cast while you stab them to death. It is strongly recommended that you get the 4-3 boss' demon soul, rescue Saint Urbain, and trade that soul for this miracle. It makes that entire world much, much easier. Just don't use it in co-op or pvp, since it can cause serious lag.
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.
The Item duplication glitch. You can take a Demon's Soul that gives 60k and easily get 198 of that item in one duplication attempt. That's roughly around 12 million souls, in which point you can pretty much raise most of your stats pretty damn high and and a good chunk of the game that gave you grief before is now all a pathetic joke. Oh and you can dupe anything. And by anything I mean ANYTHING. Need some more shard of hardstone to make your sword a godtier weapon fast? No problem we'll just give you 200 of that item easily. Need some greystone to change your shield into something else? That's no problem either. Effectively if you use this glitch in the right way, the game isn't even a fucking hard game anymore. You've just made a game infamous for its difficulty seem like something that was made for children.
Goddamned Bats: 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.
Good Bad Bugs: There is a small glitch in the Tower of Latria stage you could take advantage of so you only have to fight one of the two Maneaters.
Also, there is a bug that you can use to kill False King Allantbefore he even starts moving.
The item duplication glitch and the soul duplication glitch. The latter have been fixed in the latest patch, but can still be exploited by deleting the installation data.
Hell Is That Noise: The extremely jarring sound of the Mind Flayer's bell ringing in the otherwise silent world of 3-1.
Not to mention the ominous-sounding female voice (that turns out to be the voice of the Former Royal's Wife) that randomly starts wailing from time to time with no warning.
Also can be considered for Rydell for first-time players in 3-1. Hearing someone yelling and begging you to help them when you can't see them made this troper very nervous.
The Old Hero's roar when the player attacks him. This also acts as a warning - shoot him while he roars, and the results won't be pretty.
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.
Despite the great seriousness that the Monumental's speech to you should be taken with, it's kind of hard to do because it has a noticeable lisp.
The Old Monk's ridiculous hat. It makes him a whole lot less creepier.
Nightmare Fuel: A few areas around Boletaria; the Prison of Hope especially, what with the mad prisoners, the royal mistress' singing, the iron maidens and the illithid-looking things...
The plague babies in the Valley of Defilement. *shudder*
Are you afraid of anything? If so, Boletaria has something for you. Hate bugs? Enjoy the six-foot centipedes with four faces. Does body horror terrify you? Then the Old Monk or the prisoners should really terrify you. Scared of the dark or of heights? Then the first two areas of the Tower of Latria will make it hard for you to sleep.
Paranoia Fuel: 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 is to keep on killing them for hours on end and hoping for the best.
That One Boss: 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 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 Dragon God, the boss that comes after him, is almost an anti climax boss fight in comparison...
That One Level: 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.
Note that, if you blow it off, you can get a Composite Long Bow and about two hundred arrows (maybe 300 to be safe), then kill the Dragon. The catch is that the fastest way to get a Composite Long Bow is to go to area 4-1 (The Shrine of Storms), in which the Skeletons will rape the unprepared. What makes it worse is that to get cheap arrows, you need to rescue Graverobber Blige (his arrows are 10 souls each, while Boldwin's are 20 each) before you leave. To top it off, Blige never goes to the Nexus, so you'll have to go back to the Shrine of Storms to buy more from him. If you kill the Adjudicator though (assuming you manage the black skeleton before him), Blige will move to a much easier to reach spot, where you can buy the same stuff and more with less effort.
World 3, 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.