It's not a very good idea to start an argument over which game in the series is the best, and that includes Bloodborne and Demon's Souls (and maybe Sekiro: Shadows Die Twice). All 3 games (5-6 counting the "unofficial" entries) have their supporters, and arguments between them can get quite heated.
The Invasion PvP mechanic is also a common source of contention. It takes a few different forms, but many of them involve other players forcing their way into others' worlds without their consent for the purpose of killing them and hindering their progress. Several game mechanics actively reward this behavior, from offering useful items to advancing in various covenants. There are existing ways to play around it, mostly involving not invoking a powered-up state without which you cannot be invaded or playing offline, but such players cannot then summon either NPC allies or other players to help them either, and in the latter case don't enjoy messages and encouragement from other players. This is commonly regarded as actively rewarding griefing, especially since some forms of it actively encourage high-level players to attack lower-level players. There are even a few people who are aware of this mechanic in Dark Souls and refuse to play the series because of it. Those who do it tend to be very, very defensive, though.
Complacent Gaming Syndrome: Players dedicated to PvP in the level 120 "meta" range will usually rely on the so-called "quality build" (40 to 50 Vitality, 30-40 Endurance, 40 Strength and 40 Dexterity) and rely on the top few weapons (of the many viable weapons) that best match those builds. As a result, PvP has little variance. note Dark Souls II ups the PvP meta level range a little at 150, but has the same gist.
Darkness-Induced Audience Apathy: The games truly live up to the Dark part of their title, being set in a dying world where most of the gods have either died or gone insane and most of the human race have become mindless undead. In order to save the world, your character has to submit to burning alive for thousands of years, and even then it's outright stated to just be delaying the inevitable. Each of the games seems to make the lore even bleaker, and the vast majority of the people you meet in each game die or go insane at some point, so some people can be forgiven for not wanting to get emotionally invested in the story of the games
An odd mix of this and Friendly Fandoms with The Legend of Zelda, particularly around the divisive Wii/DS years of the latter series. With many longtime Zelda fans being disillusioned with the series for moving towards linearity, greater focus on story, and lowered difficulty, many of them came to Dark Souls for an more open experience with little handholding, a more subtle and mysterious story, more complex and difficult combat. As such, it isn't uncommon for many classic Zelda fans to name Dark Souls as "what Zelda should be." With the release of Breath of the Wild managing to Win Back The Crowd for these fans, many of them credit Dark Souls for inspiring Zelda to "return to its roots."
A straight example of this is with The Elder Scrolls. As both Dark Souls and Elder Scrolls are two of the most popular Action RPG franchises of The New '10s, fans of both tend to clash with one another.
Developed one for a while with the Monster Hunter community after the Monster Hunter: World reveal, with many a Souls fan making comparisons. Veteran Monster Hunters were not amused; While it isn't the only game to be compared due to difficulties, the former were most vocal. It was mostly one-sided, as most Souls players enjoyed the latter.
First Installment Wins: Downplayed, as there are vocal contingents who prefer the two sequels, but the first Dark Souls tends to be more well-loved and less controversial than either of its sequels, which endlessly get compared to the original because of this. Some of the more extreme fans consider the "real" Souls series to be Demon's Souls, Dark Souls, and Bloodborne (with Sekiro sometimes thrown in), feeling that From is at its best trying to build new franchises with almost entirely different design philosophies and gameplay over attempting to build on what was already good.
Friendly Fandoms: With Berserk, due to the huge influence the manga has had on Dark Souls, whose developers are fans of the series. Some fans go so far as to consider Dark Souls a better adaptation of Berserk than the actualvideo games based on the series. The fans are also quite friendly to Re:Zero, because the anime adaptation of the latter was found as a fairly interesting Spiritual Adaptation of the series with its idea of returning back after death.
Game-Breaker: There are many, many ways to heavily mitigate the difficulty of each game in the series, as explained on this page. That said, most of them don't work in every situation, and From often patches out the more egregious ones.
Genre Turning Point: Prior to the first game, third-person action games generally consisted of button-mashing until one defeated their enemies, with success in battle usually determined just by making effective use of special moves and not trying to take on too many enemies at once (and having the right armor and weapons, if the game had any sort of equipment system). Dark Souls, while not the first game to place an emphasis on learning enemy attack patterns and then effectively countering and dodging them while also eschewing any form of Mercy Invincibility, thus allowing enemies to quickly stun-lock and beat you to death if you tried button-mashing your way to victory was the one which made it the standard approach for action games in The New '10s.
Germans Love David Hasselhoff: The games are successful in Japan, but find their largest audience in Europe and North America, largely because they're strongly Western in terms of gameplay and aesthetics.
Hype Backlash: Some feel that the series' memetic status as the gold standard for Nintendo Hard modern video games ("_____ is like Dark Souls!" "The Dark Souls of _____") has gotten worn out over time. Most of these complaints come from players who have gotten used to the series' brand of challenge, as well as fans of other challenging games. This phrasing has also been used unironically by multiple game reviewers to a great deal of mockery, such as Gamesradar and Eurogamer with Crash Bandicoot N. Sane Trilogy and Spyro Reignited Trilogy respectively claiming that the games are "too hard".
Memetic Badass: Plenty within the actual games, but the series itself has become a bizarre meta example. Just about any game that's even moderately challenging will be compared to Dark Souls or be called "the Dark Souls of its genre", even games with absolutely no similarities to the series, such as Cuphead.
Misaimed Fandom: No, you're not supposed to have any sympathy for people who invade as a red. Problem is, players who set themselves up with others to "gank" are seen in the wrong.
Most Wonderful Sound: The ambient hum that is heard whenever you enter a new location or light a bonfire. In the latter's case, this is exceptionally soothing to hear after going through That One Level and finding a bonfire right afterwards.
Play the Game, Skip the Story: While the trilogy does have a complex story, many chose to play through the game instead since the story is not something that's outright told, most of it being in the item descriptions, the big draw mostly being the challenging gameplay.
Popular with Furries: Despite the game series being a depressing Dark Fantasy, it still gets its own fair share of furry fans, with prominent examples being the Bell Gargoyles, Capra Demons, and especially the Dragonoidforms. Said Dragonoids are especially prone to receiving Self-Fanservice from furry artists, most notably Dark Souls III's incarnation.
The Company of Champions in the second game is explicitly built to be this. After the (many) warnings the game presents you with, joining the associated covenant will give enemies boosted damage and health, infinite respawns, and will disable co-op until you abandon it.
"Castlevania/Belmont Runs" a.k.a. Whip-only runs (which the game admits flat out on the weapons description is a Joke Weapon almost useless against anything with armor) are common. Less ridiculous in the second game, where whips become a Lethal Joke Item on Dexterity builds if you can keep your foes at a distance.
The famed "Guts Run", meant to emulate the main character of Berserk (no helmet, no shield, only two-hand melee weapons, only crossbows as ranged weapons, no spells outside of ranged fire spells, throwing knives and emergency dagger allowed).
The Soul Level 1 run, which is completing the entire game without ever leveling up.
Sidetracked by the Gold Saucer: Even before the actual gameplay! Due to armor stats only serving a purpose in extreme cases, people tend to make various cosplay builds, but spend a good deal in the complex character creation.