Warning: Unblocked spoilers ahead! As the Land of Mists is a setting built upon suspense, players looking for creepy surprises in a Ravenloft campaign should not read farther, lest they Be Disappointed By The Revelations.
Clever Crows: Just about the only potentially good-aligned creatures in Ravenloft, they sometimes help The Atoner.
Failure Is the Only Option: Initially, Ravenloft was a one-adventure module with Strahd as the Big Bad, whose defeat by the hands of the PCs was the conclusion. Since then, the setting evolved and Strahd is still alive and well (or should we say undead and well). The Canon timeline of Ravenloft even says "528: powerful heroes assault Castle Ravenloft and are killed".
Also in keeping with this theme, certain aspects of the rules tended to make victory nearly impossible or at least very, very difficult. Most darklords had various means of resurrection that were almost guaranteed to succeed (Azalin's phylactery, Strahd's contingency spell to teleport to a hidden coffin in the mountains if he was ever killed, Harkon Lucas' mind jumping to any nearby wolf when his domain is covered in them, etc) unless the players were extremely clever. Likewise, a closed domain border pretty much meant the players were trapped, and any mundane or magical attempts to get out would fail automatically.
Finally, a trend that ran through many of the adventures involved the deaths of PCs as integral parts of the plot, usually as an excuse to resurrect them later as monsters or so the villain could force them to do something. Hour of the Knife involved any player who wandered away from the group getting killed 'off camera' by dopplegangers who promptly replaced him, Adam's Wrath forced the PCs to die so their brains could be transplanted into golems, From the Shadows had Azalin behead the PCs to fulfill a prophecy...the list goes on.
The Fair Folk: The shadow fey, of the Shadow Rift. These range from benign pixies to savage redcaps to cruel sidhe lords, all psychically manipulated by an Eldritch Abomination.
Fantasy Kitchen Sink: Ravenloft could be considered a "Horror Kitchen Sink". It was originally meant to be a Gothic Horror setting, and for the most part mostly is, but it also incorporates elements of Cosmic Horror and other horror genres as well.
Fisher Kingdom: Darkon will, over the course of three months, rewrite your memories so that you think you are a native. It's easily reversable though-one day outside its borders, and one migraine later, your real memories will come back. This is a lesser version of Azalin's curse, which prevents him from learning new spells.
Fluffy Tamer: Paladins in other settings can eventually summon an intelligent good-aligned steed or other magical beast to serve as a mount. In Ravenloft, the Powers send you a mount that tends towards being an Evil Counterpart at worst, a Shadow Archetype at best (for instance, a Paladin whose besetting sin was Pride might end up with a horse that tends to kick stableboys and might even go so far as to bite an important person who the Paladin would consider beneath them if they weren't trying to keep up their heroic vows).
Fog of Doom: The Mists that often pluck people from other worlds and deposit them into Ravenloft.
For the Evulz: Averting this trope was one of the setting's original selling points, as most of its villains were among gaming's most complex, three-dimensional NPCs of their era. Played straight in a few cases, such as Falkovnia's Vlad Drakov or Tepest's Three Hags.
General Failure: Vlad Drakov. The first time he launched an invasion, his armies were devoured by the undead. He's launched at least three more, with similar results. It's part of his curse as a Darklord to want to be a great conquerer and be surrounded by those too strong to conquer.
The Grim Reaper: The minor darklord Death claims to be this, but in reality, it's a former servant of Azalin's.
Grim Reapers are actually a form of monster that appear to collect the life force of the dying and powerful. Defeating one causes the target in question to have a miraculous recovery.
Haunted House: Several of them, the module Bleak House being the best example.
He Who Fights Monsters: Rudolph Van Richten, author of the guides to Ravenloft's monsters, is a subversion: He started out angry and vindictive, but mellowed out as time went on, even learning to forgive the Vistani who had stolen his son. He still has nothing but vitriol for liches and willing vampires, but in those cases, they were evil before they became monsters.
Although one of the possible endings for the final module suggests that Van Richten finally became a darklord in his own right.
Or was locked in an oubliette within the Mists, if the 3E product-line is accepted as canon.
He Who Must Not Be Named: Lord Soth became such because of the copyright issues and is mentioned only as a "Black Knight" or a Blackguard. In Secrets Of The Dread Realms, it is said that "no Sithican will speak his name for fear of calling him back from whatever abyss claimed him".
To a lesser extent, many of the gods such as Bane and Lathander got renamed to the more generic The Lawgiver and Morninglord.
Hereditary Curse: Some of the noble families are saddled with these, such as the propensity to madness displayed by the Hiregaard clan in Legacies of the Blood. One of the most powerful spells introduced as part of the setting allows the caster to inflict this trope upon an enemy and their descendents.
Horny Devils: The setting has relatively few demons (well, for D&D anyway) but one of the more prominent, the Gentleman Caller, is an incubus who has fathered one pseudo darklord and a couple of younger villains of the setting...
I Don't Like the Sound of That Place: Many of the lands. Convincing prospective tourists to visit beautiful "Darkon," "Dementlieu" or "Ghastria" is probably something of a challenge.
Identity Amnesia: Do not stay in Darkon longer than a few weeks. You did? Oh, never mind, of course you can stay, because you're suddenly convinced you were born there.
Ignored Epiphany: Lord Soth is the poster child for this in Knight of the Black Rose. Some of the other darklords seem to do their best to avoid confronting and dealing with the truth as well.
Strahd, ever notice that ever since that night, a reincarnation of Tatyana shows up every generation? It's only been...how many centuries?
Notice it? Strahd anticipates it. What he's refused to notice is that her repeated appearances — and repeated tragic deaths — might be trying to tell him something.
Arguably, if Strahd, Soth and all the others were the sort of people (or monsters) capable of letting go of something or compromising their ambitions then the mists would never have chosen them. Also arguably, this is a theme that ties back into the gothic genre in general, in which the villain refuses to let go of something or someone long after they should have.
It's stated in some of the source material that this may be intentional on the part of the Dark Powers. To prolong the suffering of the Darklord, the Powers will always ensure that a glimmer of hope remains: It is never solely a twist of fate that prevents success, but also a failing on the Darklord's part (or the intervention of another). Thus the Darklord will always be left with the hope that "next time", it will be different, as long as they don't make the same mistake again (of course, they will simply make a different mistake).
Infant Immortality: Subverted, new mothers need to keep a VERY close eye on their babies. And even that's not always enough.
Informed Flaw: Each Darklord has a curse of sorts, but some of the curses are abstract or far-reaching to the point where they have little practical effect in the scope of a typical adventure. One darklord's curse is that he cannot learn any new spells, but the game designers struggled to reconcile this with his Informed Ability of being a top-notch spellcaster. Later game supplements all but tell Dungeon Masters to ignore the original curse.
Azalin is a top-notch spellcaster. It's just that he's only top notch at the spells he currently knows and will never be able to expand beyond that. From time to time, his stats and spell lists are updated to take into account new spells created in those supplements. In effect, they are a Retcon of the stats released in earlier products. As for the curses of other darklords, they are not usually there for PCs to take advantage of in battle, but instead are there for the darklord to live a life of 100% pure sucktitude.
Innocence Lost: A game mechanic. Every character has an "Innocence" stat which provides limited protection from the Dark Powers. The problem is, witnessing the horrors that occur in Ravenloft tends to erode it over time, even if the character lives a life of unparalleled virtue.
Ironic Hell: Each domain is tailored to its darklord. All are given great power but at the cost of what they truly desire. Strahd is an immortal warrior-wizard and unquestioned ruler but can never possess the woman he loves. Azalin rules the largest domain in Ravenloft and is probably the most powerful Darklord there is, but cannot learn new magic. Others include a werewolf who hates his human side but will become human if he acts like an animal, a sadistic killer whose presence removes pain and whose touch cures all ills (including death), and a wolfwere who wants to rule his domain but whose people acknowledge no ruler.
Jerkass Genie: The MO of the Dark Powers is to grant a person's wish in the most horrible manner possible, landing them in an Ironic Hell.
Jerkass Victim: A number of people in this setting have bad things happen to them, but are such bad people it's hard to feel sorry for them. For example, the Vistani being eaten alive by zombies after selling an innocent man's son to become a vampire.
Mad Doctor: Doctor Mordenheim (a Captain Ersatz for Doctor Frankenstein), and Doctor Heinfroth (keeper of the resident Bedlam House). Darklord Frantisek Markov (a Captain Ersatz for Doctor Moreau) has no medical training, but he doesn't let that stop him experimenting on any animals— or humans— unfortunate enough to cross his path.
Magic Powered Pseudo Science: It is possible for non magic using characters to create Dread Golems or even Undead, but it is stated that these creations gain their dark lifeforce from the Dark Powers granting it to them.
Magical Library: The lich-king Azalin has a giant book which houses the self-updating life stories of every sentient being who has ever been born in his domain of Darkon, or who's entered it and stayed long enough to lose all memory of their previous life, in addition to Azalin's continuously updating biography. Crossing out or rewriting your lifestory are among the few ways to recover from Darkon's insidious Identity Amnesia effect, as is asking Azalin to rewrite said lifestory to your benefit.
Never My Fault: This is a failing that a lot of the morally greyer (or outright black) are prone to.
One of the big flaws of the Vistani, as a whole, is this; it's all but outright said that it's only a crime if it's done by a giorgio (outsider) to a Vistani, not the other way around. The absolute perfect example of this is the Vistani wise woman cursing Van Richten to bring a horrible death to anyone who comes to consider him an ally for destroying her tribe... not once considering the fact they brought it on themselves by kidnapping his son and selling him to become, essentially, a vampire's catamite.
Making things worse, the Vistani who died and was used as an excuse to kidnap van Richten's son? He got his injuries in a failed attempt to kidnap somebody else's son.
No Campaign for the Wicked: Even if there is no strict obligation for Player Characters to always be goodie do-gooders, morally questionable actions tend to attract the attention of the Dark Powers. After five failed "Power Checks", the character becomes an NPC; the sixth sends them headlong into darklord territory.
Not So Different: The setting goes to pains to describe that, as often as (insert group here) is regarded with distrust and suspicion by others, they are often just as bad and may genuinely give their neighbors reason to treat them the way they do.
The Vistani are a great example of this. On the one hand, the "giorgio" are clearly shown to be xenophobes. On the other hand, the Vistani are, in many ways, just as xenophobic, as well as being smugly self-righteous, indifferent to cultural differences, and prone to blithely committing acts of theft, kidnapping and worse against giorgio simply because "it's not a crime if we're doing it to them".
Ominous Fog: The Mists of Ravenloft are iconic setpieces, and are out and out said to work for the Dark Powers. The Mists are often used to block various domains from each other, and to transport unsuspecting characters to wherever the GM pleases.
Our Demons Are Different: All evil outsiders usually come to the demiplane by possessing someone, or by being summoned in by powerful spellcasters. They become trapped like everyone else once they are in. A powerful Incubus named the Gentlemen Caller is trying to escape by creating a child capable of escaping from the Demiplane. Fiends even got their own rules supplement dedicated to them called Van Richten's Guide to Fiends in 2nd Edition.
Due to their rarity, Van Richten himself is under the impression that All Demons Are Different, i.e. every one is unique in its powers and appearance.
There are also plenty of conventional D&D goblins in Ravenloft. Even they are Different from most game goblins, being closer to their fairy-tale roots.
Our Monsters Are Different: The iconic creatures, as described by Rudolph van Richten in his monster hunters' guides, have unique features that must be researched in order to properly fight them.
Our Orcs Are Different: To set it apart from traditional fantasy settings, there are actually no orcs in Ravenloft at all. There's a record of one single orc who was brought to the Land of Mists as part of a twisted carnival, but the Dark Powers warped him into a grotesque ape-man.
The setting thus doesn't have half-orcs as Player Characters, but Calibans — humans deformed in their mothers' wombs by curses or evil magics — fill that role.
The setting has literally TONS of variants (even though the regular ones are most common): a racial variant for each race (e.g. elven vampires kill vegetation and are harmed by moonlight), nosferatu that can walk around in the daytime, vampyres (who aren't undead but living creatures that suck blood), and about a zillion others.
Path of Inspiration: The Eternal Order, Darkon's official religion, is a Religion of Evil in disguise and a tool of political control of the population in the hands of Azalin.
In G'henna, the Church of Zhakata also fits this trope.
Popularity Power: In Knight of the Black Rose, Count Strahd lures Soth into attacking Gundarak, a neighboring domain, on the promise that a portal there may lead him back home. Soth singlehandedly breaks into Duke Gundar's castle, murders his son, and uses his blood to open the portal while the Duke, supposedly an all-powerful Darklord in the seat of his power, takes one look at Lord Soth and vanishes until the scary man goes away. When the portal doesn't work as advertised, Soth intends to do the same to Strahd, but gets sidetracked by the Sorting Algorithm of Revenge and trapped in his own domain before he get the chance.
Partially justified in game stat terms as Soth was both higher-level and a Death Knight, a very powerful type of undead warrior who as such was impervious to most of a vampire's bag of tricks.
Powers That Be: The Dark Powers. No one is sure what they want, or why. Only that they tend to do dramatic things, and stay strictly off-camera.
Railroading: A lot of early Ravenloft adventures forced the party to go from Point A to Point B. Some even recommend the party die in order to get where they need to go. Also, the nature of the setting makes this very, very easy for a GM. Characters going in the wrong direction? Have undead hordes keep attacking them out of nowhere — it happens all the time here. They want to visit another domain? Just close it off, problem solved.
Razor Floss: Spider-like monsters called Head Hunters spin razor-sharp near-invisible webs that can decapitate the unwary.
Religion of Evil: The setting has quite a few of them, evil cults and dark religions being quite a natural aspect of gothicism, but the most notable one is the Church of the Lawgiver, which is the state religion of both Nova Vaasa (which is generally regarded as a self-centered backwater, even in Ravenloft) and Hazlan (where the Mulan desperately cling to it as a further way to control the Rashemani who they persist in treating as subhuman chattel despite the slaves outnumbering the masters by about nine to one). Being that the Church of the Lawgiver is basically the worship of Bane, Faerunian God of Tyranny, with the serial numbers filed off, it's not surprising that its membership is comprised solely of Lawful Evil and Lawful Neutral types — its dogma revolves around preaching that Status Quo Is God, those who are socially inferior must obey, and rebellion against one's superiors in even the slightest way warrants eternal damnation in the Hell of Slaves.
The Renfield: Naturally, since the setting has every classic vampire trope, there's plenty of these running around.
Roaring Rampage of Revenge: A lot of these get mentioned in the background lore, and it's possible to portray a campaign as basically an extended one of these against the darkness.
Van Richten himself had one of these. When the Vistani kidnapped his son because he was unable to keep their fatally wounded tribesmember from dying, he went charging off into the night in blind pursuit of them. A chance encounter with Azalin Rex, the Lich King of Darkon, and Van Richten found himself leading an army of zombies, because the lich was amused by it. These enabled Van Richten to trap the Vistani and force their leader to tell him where his son was. When she gloated they had sold him to a vampire, who had already transformed the boy by now, Van Richten set the zombies on them and slaughtered the lot of them,
Safety In Muggles: In the module Death Ascendant. While the religious service in the temple is going on the Kargat members take aside worshippers one at a time and drain them of their life force. When they try to do this to a PC, if the PC protests the Kargat member backs off because they don't want to make a scene.
Sins of Our Fathers: Entire bloodlines can be cursed. In one or two realms, Darklord status is passed on generation to generation.
So Beautiful, It's a Curse: More literal than usual - if a Ravenloft woman is gifted with unusual beauty, it's generally so the Powers can torment her with particularly vile suitors that think No Means Yes and Rape Is Love. Or, alternatively, torment her by gifting her with supernatural beauty that causes her to kill or repel potential soulmates.
Inverted for the darklord of Dementlieu, who is (literally) cursed to appear uglier and uglier to any woman, the more strongly he's attracted to her.
Super Weight: Even a minor darklord of a small domain is going to be at least a 3. Major ones are the high end of 4, and would be 5 except the Powers regularly remind them exactly who's in charge here. As for the PCs, they'll be whichever tier the Powers decide for that adventure and like it.
Tailor-Made Prison: The entire setting is just a grouping of Oubliettes for the Darklords, along with all of the people who have to live in the domains, or those who get pulled in by the Mists from other worlds.
Replacement Love Interest: There's an unfortunate Dhampire-like creature called a vorlog that is created when someone is in the process of being turned by this ritual, but the vampire is slain before the victim is fully transformed. They keep trying to recreate the aborted bond with the vampire through (non-vampire) surrogates. It never ends well.
Unstuck in Time: Castle Forlorn is an example of a place that's unstuck in time. While the wider domain of Forlorn is temporally concurrent with the rest of the demiplane, people who enter the castle can find themselves shuttled between at least three different eras in its history.
Unwitting Pawn: Darklords and especially vampires like arranging these. There's a very good chance that by the end of a module the players will end up fighting whoever asked them for help in the first place. On a larger scale, the Powers are the ones that are really "playing" Ravenloft — all the characters, PC or not, are basically toys for their amusement.
Uplifted Animal: Urik von Kharkov is a panther-turned-human-turned-nosferatu. Also, the Wildlands do this to animals that stay there for long, and Markov cranks them out via vivisection.
Van Helsing Hate Crimes: Pretty much averted here; the premiere hunter of the setting, Rudolph Van Richten, is usually quite sympathetic to monsters who honestly have no choice in the matter, saving his vitriol for ones that like what they are.
At least one adventure, however, has the real villain try to convince Van Richten to commit these crimes against a group of vampire wannabees by convincing him they are real vampires, and explicitly say he will unless the PCs stop him.
Vegetarian Vampire: Averted for animal blood; even vampires from other worlds who'd previously been subsisting that way have to turn to human victims or starve. Some vampires use "shallow feeding" on multiple victims to avoid killing each night, but this is generally done for convenience (e.g. Strahd's feeding on captives in Castle Ravenloft's dungeon) rather than mercy.
It is noted that this was done deliberately: If the world is pitch-black, the actions of the heroes will shine all the brighter. Despite its World Half Empty aspects, it's actually pretty far along the idealistic scale.
Debatable, since canonically, the heroes can't do much to actually change anything. You can never get rid of a Darklord unless the Dark Powers allow it (which they won't), at best you might (temporarily) save half a dozen people out of millions that are suffering.
But to paraphrase the old parable about starfish, it still makes a difference to them.
You can get rid of a darklord, actually. And in at least one published adventure, you do.
The Domains of Dread campaign book and 3rd Edition products from Arthaus both eased off on this trope, as these products adapted the setting for native player characters, not Mist-imports from other game settings. Rather than a blatant Crapsack World, they made it feel more like an entire Demiplane With A Dark Secret, in which the masses are largely unaware that their world is any grimmer than normal, the average person never knowingly meets a monster, and the existence of darklords is unknown to all but a handful of occult scholars.
Year Inside, Hour Outside: Time in the Shadow Rift flows much more quickly than it does in the rest of the Land. Also, Baron Evensong's curse is to be trapped in his parlor every night, which lasts 100 years for the room's occupants.