Absurdly High Level Cap: The maximum level is 675. According to developers, the limit only exists because of integer overflow problems on Experience Point values, and if anybody were to actually reach level 675, experience would be re-implemented using floating point math, removing the cap.
Booze-Based Buff: For most people, being drunk increases healing/recovery rate a bit and increases both pain tolerance and pleasure tolerance (you can be stunned by sufficiently pleasurable sensations), but for the Brute Squad, it's the stuff of life itself, making them stronger, faster, better.
Breaking the Fourth Wall: Some NPCs are aware that some of the people found in their world are the puppets of beings from an entirely different order of reality — that is, Player Characters. One has written a book about it.
Breath Weapon: A number of species breathe fire by way of phlogiston glands. Esoteric breath weapons are explained by phlogiston absorbing magical essences that infuse the glands' owner.
Came Back Wrong: Death, rather than resulting in loss of XP as typical of a MUD, causes random trauma effects, such as mental disorders.
Canis Latinicus: Lots of this flying around, especially in the names of the mage guilds. Sometimes aggressively averted, as with the association Collegium Magistrorum, which name features the correct declension of the noun "magister" in the plural genitive case.
Demonic Possession: The Kazarak guild. The process of "joining" this guild actually involves your character dying as its body is converted to a geomantic engine, after which you are playing the Kazar demon walking around in your former character's body.
Died Standing Up: A trait called "thanatogalvanism", naturally occurring in such notable species as chickens and giant chickens, as well as achievable by those in a berserker rage, results in one's body continuing to move, and even spasmodically fight, past the nominal point of death, including by decapitation.
Difficult but Awesome: Various guilds, particularly the Aligned, artist-philosopher-magicians with a huge array of abilities that work by highly esoteric logic, as well as the entire game, which doesn't have quite the learning curve of Dwarf Fortress but gives it a run for its money.
Easy Amnesia: Probably averted, though it's debatable. Both retrograde and anterograde amnesia occur, with anterograde amnesia having a Memento-style subtype referred to, again debatably, as Korsakov's syndrome. The possible aversion is that both temporary and potentially permanent forms are observed, though even nominally permanent mental disorders are curable by magical means.
Easy Exp: Exploration experience; i.e., experience for wandering around gawking.
Extreme Omnivore: Shoggoths are "organivores", able to digest anything with a significant carbon content that's not entirely acid immune. Since this includes most living creatures (shoggoth digestion can be a viable combat maneuver), they can and will eat things ranging from wooden barrels to live ogres.
Eye Beams: Used extensively by the bezhuldaar (read "beholder") race, and found in a number of other instances; the demonic fomor race has heat-vision-like eyebeams.
Finishing Move: A number of guilds have instant-kill powers that can only affect mortally wounded opponents; players and developers alike generally refer to these as finishing moves, though there is no formal categorization involved. Some are up to Mortal Kombat levels of violence, like the Ringwielder rune "arkonaad", which causes the victim to explode with sufficient force that its flying limbs can cause serious injuries to bystanders.
Food as Bribe: The most pleasant way of getting past Kerberos, the three-headed dog guarding the fiery portal from the cave of Tainaron to the River Tethys.
I Call It "Vera": Nicknaming support allows you to call your weapon, or nearly anything, whatever you like (and automatically continue to perceive it by that name).
Improbable Weapon: Basically any physical object is wieldable as a weapon, but yo-yos (note that the yo-yo was actually originally designed as a weapon), magical flowers and umbrellas exist in particular, as well as weapons that randomly shapeshift into odd forms. Players who achieve extreme size and carrying capacity have demonstrated a fondness for wielding the Losthaven clock tower as a weapon.
Item Crafting: Not that much of it, actually, but here and there.
Ki Attacks: Sufficiently enlightened Aisenshi gain access to the chi punch and chi kick maneuvers, unarmed attacks that do a combination of physical and energy-based damage, not to mention leaving a sparkly trail behind in the air.
L Is for Dyslexia: Played straight with a dyslexia disorder that manifests as read text displayed to the player having characters rearranged and rotated (though the implementation doesn't go so far as Spoonerisms).
Last Chance Hit Point: Being at zero HP (on a relevant limb, such as your head) is okay, it's negative HP that's trouble. In possibly an expansion of this trope, you can endure a degree of negative HP on your chest, but are mortally wounded and bleeding out while this is the case.
Last Lousy Point: Skill development slows down the closer you get to your maximum, making the last point exponentially harder to get than most of the rest.
Luck Stat: Luck was once, but is no longer, one of the core character stats, or "attributes". But luck is still around, now as a "trait" that a character may or may not have (and may be good luck or bad luck).
Ludicrous Speed: Seen in the Phylactery of Blinding Speed, a cursed item that makes you a lot faster... to the point that the vision centers of your brain can't process the incoming information any more and cease functioning.
Made of Explodium: The "clockwork orange" monster (oh hey, wonder if that's a reference), a sort of self-guided living bomb.
Magitek: Steampunk power armor that runs on fire and water elementals.
NPC Scheduling: Not universal, but a number of settlements have NPCs who, for example, run a shop during the day and go home at night. Tends to lead to more griping than anything, except in cases where the shop also has a night shift.
Omnicidal Maniac: The dead god Scyros. His omnicidal mania is why he got put down (in some of the better-developed setting history).
One Size Fits All: Usually averted, as clothing and armour must be the right size in order to fit, but some items magically resize themselves to fit you.
Olympic Swimmer: People routinely swim between continents, though flying is preferred because it's immensely faster. Trying to swim between continents wearing plate armour may not work out, though.
One-Gender Race: There are a few of these, starting with fantasy staples of male-only satyrs and female-only nymphs. In less prosaic fare, Kali has a servitor race, the dakshina, who only seem to come in a female model, and the aethoss tree-folk are all hermaphrodites (though this implies considerably different things in a plant lifeform than it would in a human).
One-Hit Polykill: Probably best seen in the Ringwielder rune "zerivalak", chain lightning.
Organ Drops: Organs can be cut from corpses. Players, apparently all channeling their inner 13-year-old boy, cannot seem to get over their amusement at one of said organs being the anus.
Our Werewolves Are Different: The Lupines, a guild that can only be joined by the garou (wolf-people) race, and which enables them to shift to a variety of forms comprising varying components of human and wolf. Clearly based on Werewolf: The Apocalypse, using some of its terminology, but goes off in its own direction.
Petting Zoo People: Playable races include cat-people (sekh), wolf-people (garou), minotaurs, fish-people (urlnu), bug-people (invae), and spider-people (rachnei), among others.
Player Versus Player: There is a dedicated PVP group, the Soulburners, though it doesn't see a lot of action.
Praetorian Guard: Once upon a time, the Millenarian Guard, imperial guard of the Altrian Empire. Their remaining legacy in the present day of the setting is the spectacularly enchanted armour that was reserved just for them.
Pre Ass Kicking One Liner: The Wandslinger Litany, based on the Gunslinger's Creed from The Dark Tower, adapted for a slightly more medieval sort of fantasy setting. Wandslingers recite it in order to enter a Tranquil Fury state called "Wandslinger focus". If you hear this, duck:
I do not aim with my hand. He who aims with his hand has forgotten the face of his father. I aim with my eye. I do not shoot with my hand. He who shoots with his hand has forgotten the face of his father. I shoot with my mind. I do not kill with my wand. He who kills with his wand has forgotten the face of his father. I kill with my heart.
The gendered pronouns will automatically change to match the speaker's gender. In a further homage to a particular Crowning Moment of Awesome from The Dark Tower, if the Wandslinger currently suffers from the mental disorder Tourette's Syndrome, the last line is replaced with, "I kill with my heart, motherfucker".
Scaled Up: Seen in the Rogue Serpent quest in Corna.
Schmuck Bait: There are several locations where cruising around without reading the descriptions has results like barreling off an underground cliff or getting turned to stone by the basilisk in the next room.
Scratch Damage: Averted. Armour doesn't make you less likely to get hit, it absorbs damage.
Sssssnake Talk: Sentient reptilian species tend to display this. It isn't applied when they speak in their own languages, as extended sibilants are assumed to be an ordinary part of those, so it's mainly seen when snakey types talk the local equivalent of English, Anglic.
Standard Status Effects: Poison, blindness, silence, paralysis. Could be considered averted by the addition of deafness, anosmia, ageusia, anhedonia, and an entire host of mental disorders.
Starfish Aliens: The lloigor, the first beings to come into existence, of godlike power and formed without any relationship with human-scale beings (as opposed to the kathexoi, the beings to whom the term "elder god" best applies, corresponding to the Titans, who had some sentient feedback in their origins, and plain old gods, who are shaped mainly by sentient consciousness). Only a few of them are capable of things like language use or anything a human would recognize as thought.
Status Line: "Show vitals", also known as "hp", plus an entire customizable status bar faculty.
Stuck Items: Some cursed items display sticky behavior. Hijinks ensue when players decide it is more cost effective to have their buddy hack off the hand with the cursed ring on it, then reattach the limb, than to get a remove curse spell cast on them.
Subsystem Damage: Limb-based hit points, and one of the sources of mental disorders is getting smacked in the head.
The Undead: You bet. Undeath is handled as a general term for any situation where a being's continued existence is being sustained by processes other than its natural biology; player characters have a number of options for becoming undead.
The Unintelligible: A mental disorder, productive aphasia, converts player characters and NPCs alike into this. Hilarity ensues.
Viral Transformation: In a local implementation of NetHack's "eating a mind flayer makes you smarter", eating the neural tissue of the vlekthid (read illithid) or gogtzul (read Star-Spawn of Cthulhu) races results in that neural tissue colonizing your own nervous system, making you smarter and less emotional, and giving you both psionic powers and often-crippling mental disorders.
Visible Silence: If you can hear part of what somebody is saying, the parts you can't hear are converted to ellipses.
Welcome to Corneria: NPCs are not generally great conversationalists; mostly when they have anything useful to say, the out-of-character "info" command shows you what verbal cues they will respond to. (An exception is that in several towns, NPCs can be asked for directions to interesting places in the town in a fairly freeform fashion.)
Well-Intentioned Extremist: Arguably, Kalyxes the Shadow Lord, who imprisons people he has determined to pose a threat to "balance", based on magical divination — that is, he locks them up because of what they otherwise would do, in a fantasy version of "precrime" a la Minority Report.