Anticlimax Boss: After reaching the Fortress of Regrets, opening the Bronze Sphere, and discovering both your true name and the source of your torment, thereby achieving emotional catharsis, you get to fight the Big Bad. While you can defeat him with words if you have high enough wisdom, intelligence, or charisma, or if you brought the Blade of the Immortal, he isn't all that impressive of an enemy in physical combat. Also, resurrecting Vhailor and telling him the truth of your identity will make him an unbeatable killing machine that drops the boss with relatively few hits.
Complete Monster: The worst monster is a former incarnation of The Nameless One, known simply as "The Practical Incarnation." Combining a wicked intellect with a lack of morals, the Practical enslaved an adventuring party to him, most notably by convincing a githzerai to follow him based on a religion the Practical invented. The Practical manipulated and murdered countless people, but the worst was what he did to the one woman who deeply loved him: after winning her love, he murdered her so her ghost, bound to the material plane by that love, would remain as a guide for future incarnations should the Practical's quest fail. At the end, the Practical confronts the current incarnation The Nameless One and attempts to absorb him into the Practical's own consciousness along with all the others so the Practical can achieve his goals.
Cult Classic: Essentially doomed to this, given its very unorthodox design and focus on text.
Enjoy the Story, Skip the Game: It's noted for its excellent story, characters, and loads of freedom and roleplaying options, but combat is not the game's strong point. However, you can solve most problems with dialogue options rather than fighting (the entire game has a total of three unskippable fights, and while most areas do have regular enemies who will attack you no matter what, they can be avoided if the player so wishes).
The plot itself can give birth to umpteen, but in-game dialogue mentions an all-time great: that the Lady Of Pain, mysterious and godlike ruler of Sigil, who can banish lawbreakers to endless, eternal mazes (or just kill them by casting a shadow made of thousands of magical blades) for crimes like murder, treason or daring to worship her, is actually six giant squirrels wearing a cloak, a ring of levitation and an illusion spell.
A popular fan one is that the Nameless One's original incarnation is Zerthimon. Despite the fact that the Nameless One appears as a human male, albeit covered in scars (not only on screen, but characters who bring up the Nameless One's appearance also describe him this way), and the game's creators have outright said that, while they don't *know* who he was, they do * know* who he was not, and he was not Zerthimon.
Larval Worms are some of the hardest hitting enemies in the game, obviously meant for higher level characters...and technically level 1 in the game files despite their damage and HP, meaning you can use Cloudkill to kill them en masse in Undersigil for 8,000 experience a pop. Since Undersigil is repopulated every time you leave and come back, it's entirely easy for a player to get tens of thousands of experience points with one casting of a third level spell and repeat the process as often as they like. Fixed in the unofficial patch, resulting in much wailing and gnashing of teeth for players interested in quick and easy leveling.
Harsher in Hindsight: You remember all those times you chose to kill yourself just to fulfil a quest, like getting that 1000 copper from that lady who wanted to murder someone, or to make fun of that priest speaking about the afterlife? Yeah, someone else, a random innocent somewhere in the multiverse, took the bullet for you every time you did it.
Hilarious in Hindsight: Yeah, yeah, we know that "Vrischika" is Sanskrit for "scorpion"... but is there anyone who isn't going to think of Homestuck when they read that these days?
Hype Backlash: Over the years the game rose to the status of the ultimate western RPG ever made, if not one of the best games of its time. And no-one is denying its greatness, but it also leads to numerous new players ending up being disappointed at the game, as it didn't quite live to their (too high) expectations.
Ink-Stain Adaptation: Dak'kon, an optional party member in a licensed title, is, to this day, the most famous and iconic githzerai in the franchise of Dungeons & Dragons. Naturally, this means that from 3rd edition onward his entire race, once characterized by being chaotic and capricious, has become much more like him, event though, originally, his lawful characterization was one of the factors that had made him an alienated pariah among his own kind.
Memetic Mutation: "Updated my journal." An average playthrough will have this sound play several hundred times, and the phrase has become an in-joke between fans. Most of The Nameless One's barks have also attained minor in-joke status.
There's a chance it will be crossed in the story's narrative. You have several opportunities to cross it yourself. Some past incarnations (especially the Practical Incarnation, and apparently your first incarnation) crossed it already.
The Practical Incarnation made Deionarra love him so her soul will be bound to him in death and force her to help him if he ever returns. It was his backup plan should he die in the Fortress of Regrets, which he did.
Nightmare Fuel: Don't let the general quirkiness fool you, there are a lot of highly disturbing things in this game; enough that this trope requires its own page.
One-Scene Wonder: This game has more than a few, among them the following: Ravel Puzzlewell, meeting the incarnations, and, of course, the Transcendent One. In fact, one might argue that very few generic characters are particularly forgettable.
Player Punch: Hoo boy, yes. In particular, trying to play one of the evil paths is akin to facing down an Olympic boxer with lead weights tied onto your extremities. If the player is not a Complete Monster, of course.
If the Nameless One joins the faction of the Sensates, he gains access to their private sensorium. Within is a sensory stone entitled 'Longing'. In it are Deionarra's experiences, days before her death. And as the Nameless One, you experience both sides of the conversation (it being with the past incarnation Deionarra loved), and come to *know* its horror, especially as a good character. It is not so much a Player Punch as the Lady's Shadow of Player Punches. In the same private sensorium, you find a trap from the Paranoid Incarnation and the experience of being tortured by Ravel Puzzlewell with some interactivity. 'Longing' manages to be the worst of the three, by far.
Also, when navigating the Fortress of Regrets, you have to watch as each of your scattered party members are approached by the Transcendent One. They all die. And there's nothing you can do to stop it.
The Rubikon Dungeon Construct. While completely avoidable, essentially a Bonus Dungeon, if you want to find Nordom or kill the Evil Wizard, you have to set it to 'Hard' difficulty and go through it... and it is basically 62 instances of the same room copied over and over and over, with one to three pain-in-the-ass constructs which are completely identical from room to room. After the cuteness factor wears off, you can be forgiven for just going on a mad dash through the thing without stopping to fight.
Curst, which plays out exactly like it would in a normal RPG. It should come as no surprise that this segment of the game was made by a completely different team (though the dialogue was still written by Avellone's team), who obviously didn't "get it".