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.
More of an intentional homage than anything else. The two games have little alike other then being RPGs based on Dungeons and Dragons rules. Considering what a success the AOL Neverwinter Nights was, it's likely that a few people remember it.
In The Very Definitely Final Dungeon in the base campaign, you meet Asheera, a woman from a parallel world called Sahaladar who looks strikingly like Aribeth. She says Morag has been invading other worlds via the portals in the Source Stone, and has tempted her with psychic dreams of power like Aribeth, but she has refused. Asheera must return to her own world but gives you an amulet with some nifty bonuses for fighting the last gauntlet of Elite Mooks before departing. The encounter has no build-up, no explanation, is never mentioned afterward, and really just raises a lot more questions that had never been an issue before and that the game does not answer before you end it. Not to mention this amulet could have been given to the player is any number of alternative ways without bringing up those questions.
Shadows has a mid-chapter Interlude. Two mini-adventures occur in it before you reach your destination to continue the main plot, neither of which has any bearing on said main plot. The second allows for some brief foreshadowing of the villain's plans, but it's in optional dialogue you're likely to miss.
In Hordes of the Underdark, you come across A band of villains from the original game led by Grimgnaw, who are looking to take over Cania with Mephistopeles out of town, and they attack you since you're strong enough to threaten their claim. The fight received no foreshadowing, is not mentioned afterwards, and serves no plot purpose. It's mostly only there to load you up with powerful items for the final few bosses, the ones you don't need functioning as Vendor Trash for the upcoming Money Sink where you'll be spending hundreds of thousands of gold to get what you need.
Ensemble Darkhorse: Valen Shadowbreath is very popular with fangirls due to being a badass with a tragic backstory and a prehensile tail.
Even Better Expansions: The original campaign was okay, but had very repetitive quests, fairly simple and two-dimensional characters, and gameplay could be stilted at times. The expansions had much more creative and engaging quests, very complex and likeable characters, and new gameplay features and tweaks. As a whole, the expansions are more refined and polished than the base game and it shows.
The game hands you one at the start — the Stone of Recall. It teleports you from your current location to the nearest Temple of Tyr, where there is always a priest on-hand to fully restore your HP, cure all debuffs, and offer shop services. Furthermore the Temple is where your henchman respawns when they die, there's a portal back to where you used the Stone, and the Stone has unlimited charges each day. Any time you start to lose a fight, just use the Stone of Recall to warp away, get full healing, buy any potions you might need, then use the portal in the temple to warp back to the spot you used the Stone and keep fighting. The only downside to this? The portal costs money to use, but it's pennies in a game where Vendor Trash is plentiful. The two expansions, who have their own counterpart warping items, noticeably nerfed them.
Clerics. Wearing full-plate armor with no magic penalty? Check. Equipping most kinds of weapons by default? Check. Casting the best spells of the game by far, like Harm? Check. Doing so without even having to learn and dislearn spells like the arcane classes? Checkmate.
The "Harm" spell in itself was this throughout the game, lowering the hitpoints of any non-undead enemy down to 1 (and anyone with Harm also gets Heal, which can do the same thing to undead as well as healing themselves if needed). It's a sixth-level-spell, meaning you get it at level 11 and will just keep on using it forever all the way to level 40. This particular brokenness was, of course, imported from 3rd edition D&D.
In Hordes of the Underdark, take Sorcerer classes until you get all three auto still spells, then 10 levels of Dragon Disciple, then the rest paladin. The result? A killing machine that slaughters its way through a literal army of demons effortlessly. Now add the fact that you an easily get Enserric the longsword to +10, and find an amulet of regeneration early in the game...
Five words - Maximized Isaac's Greater Missile Storm. To summarize, IGMS is a Level 6 Wizard/Sorcerer spell that fires magic projectiles, one for each caster level up to 20. They seek out and hit targets in the area of effect and do 2d6 damage each. Enter Maximize Spell, which raises the level of the target spell by three (making IGMS Level 9) and removes the variables from its effect, placing the values at maximum damage. This means that a Maximized IGMS will fire up to 20 projectiles, each doing 12 damage, for a total of 240 damage for one spell. Oh, and if you use this against a lone enemy (ie, a boss), all the projectiles fire at them. Add in the fact that in Hordes in particular you begin the game at Level 15 and keep going so you'll have a lot of spell slots for higher-level spells...
To put that damage into perspective, the "Epic Spell" Greater Ruin (that is, one of the top-level spells in the game) does 35d6 damage, to a maximum of 210 damage under optimal conditions—keeping in mind that a Maximized Missle Storm is guaranteed to do 12 damage a missile, while Greater Ruin you still need to rely on the luck of the roll.
Traps. You can beat the game before reaching level 10 with them by using a glitch to stack them together and killing storyline-required enemies.
As pointed out below, undead bosses often have loads of defenses and immunities as well as fear auras and buffs like stone skin, which makes them very difficult for most character builds...except for paladins who are immune to fear and can cast Lay On Hands which goes right past all of the undead's defenses. While a sword or mace might slowly chip away a hitpoint at a time on Brother Tomas, one casting of Lay On Hands can take a third of his health down.
In chapter two of Hordes of the Underdark, The Drow smith Rizolvir will upgrade your weapons for huge amounts of gold. If the player has been careful to save up gold throughout the campaigns they've played they can end up with enough gold to get all of the upgrades on a single weapon. The best possible weapon in the game with the best possible character build can be upgraded to do a maximum of nearly 300 DAMAGE PER ROUND!
In Hordes of the Underdark, Enserric the Sword can become more powerful while vampirizing the health of the Player. Wearing the Greater Amulet of Health will make the Player immune in stat drains. This very worthy combination (powerful upgradable sword and amulet immunizing from poison, diseases, and stat drains) is a game breaker.
And for added fun, play as a Fighter / Weapon Master (8/7 is a good level split), with your feat choices centering on Greatsword (Weapon Focus, Improved Critical). Tell Enserric you don't need a longsword, but that a greatsword would be a much more useful weapon. Enjoy your health-restoring slab of sarcastic oversized death.
Greater Sanctuary (6th level cleric / 7th level sorcerer or wizard spell) makes the caster ethereal and undetectable by hostiles as long as the caster doesn't do anything hostile themselves. Black Blade of Disaster (9th level sorcerer/wizard spell) creates a free-thinking weapon with some very nice bonuses that does all the fighting for you as long as you don't do anything other than move around. If you cast Greater Sancturary, then Black Blade, Greater Sanctuary doesn't lose its effect (summoning spells don't count as hostile actions). The only thing threats end up noticing is the weird-looking black thing about to slice them into easy-to-carry pieces.
Magnificent Bastard: At least Mephistopheles in Hordes of the Underdark. It's practically in the definition of an archdevil. After being imprisoned by a puny drow who got hold of his True Name, he predicts that she will possibly be defeated by the Player Character, which turns out quite true when this encourages her to bring them to her and enables the devil to slip loose of her control. He then tries to conquer the entire world in an improvised bid to become the supreme ruler of Hell by making this world its lowest level. And when the player character catches up to him, he opens by talking their companions into joining him instead. Of course, the writers' effort to make all this believable is only partially convincing, but it comes across as their fault rather than the character's. Also has a good evil voice and Evil Laugh.
Most Annoying Sound:The quiet tinkling sound of the fairy companion that threatens your sanity.
Nightmare Fuel: The Charwood. Do not adventure in this region shortly before bedtime. We mean it. It becomes more of a Tear Jerker once you find out the backstory, but it's still pretty scary.
Padding: The Interlude between Chapters 1 and 2 of Shadows is broken up into three parts, the first two of which have no plot relevance and just give you small dungeons to go crawling through so you can get back on your journey to your actual destination.
Scrappy Mechanic: Aura of Fear. Many high-level boss-type enemies, particularly Mummies and Dragons, have Auras of Fear around them that inflict the Frighten status on anyone who comes inside the aura. Characters under Fear become entirely uncontrollable and wander around aimlessly, suffering a -2 penalty to all saving throws. You need equipment to prevent Fear or a high enough Will to make your saving throw against it, and even then as long as you remain within the aura you will need to continue to roll to save against the Aura of Fear. And your henchmen? Forget them, they will pretty much always fall victim to it. Getting hit with Frightened is pretty much a death sentence any time it happens, and can make otherwise unremarkable foes into a Goddamned Boss.
That One Boss: Plenty, usually the undead bosses. Also, facing a boss enchanted with Fear (i.e. mummies, dragons) without being immune to it can make the fight extra difficult.
The first chapter of the original campaign has the Intellect Devourer. It is highly resistant to physical attacks and has mind-affecting spells. Luckily, this can be remedied early on by equipping gear that provides mind-affecting spell immunity or drinking a Clarity potion beforehand.
Chapter 2 of the original campaign has two (each is a high-level wizard lich): Brother Toras of the Arcane Brotherhood and Voleron in the Ruins of Illusk. He will continually spam high level spells on the player and followers. Unless your well prepared ahead of time Toras and crew can and often will mop the floor with you.
Shadows of Undrentide has a Blackguard Skeleton appear in a mid-level dungeon. It does not sound so bad, but it is an extreme Damage-Sponge Boss (with lots of regular damage sponge skeletons acting as bodyguards) whose undead status makes it immune to anything besides straight-up damage. They will surround you, and you will die again, and again, and again. At least Fighters can walk around in the Blackguard's fancy armor afterwards. Thankfully, you can always retreat to safety through a secret door nearby. (Unless you Failed a Spot Check) Cheap, but effective.
Also in Hordes of the Underdark is Vixthra the dracolich of Drearing's Deep. He can revive himself as many times as long as his phylactery remains intact, which is guarded by 2 skeleton golems. Vixthra himself has a huge amount of hit points and his breath weapon can do insane amounts of damage. Tank characters can take more punishment but lighter classes can have a hard time due to small amount of HP. And even though there's a shortcut door to the phylactery, the golems can gang up on you and Vixthra can even fly to your location to defend it. At least Vixthra is undead, so you can always use "Lay on Hands" as an attack if you're a Paladin/Champion of Torm - just make sure you have enough charisma to inflict all that damage... and make sure that the phylactery is broken first.
Also in the Isle of the Maker in Hordes of the Underdark, there's a room in that animates your weapons and turns them against you. It's even more devastating for those who dual-wield weapons and you're forced to use the spare weapons scattered on the floor or from your inventory. The game warns you about this, but it can also be countered by equipping a weak weapon BEFORE entering this room's floor, which the weapon spirit will detect. Reequip your weapon after entrance to the floor.
The Valsharess is a high-level spellcaster, and you have to leave your allies behind to fight her one-on-one. Due to spell resistances, if you're a spellcaster class yourself prepare for a long, drawn-out battle.
The Untwist: You mean, the Jerkass who has been ridiculing efforts to find a cure for the plague and his priests who cast "blessings" that have the appearance and sound effect of a Negative Energy spell, are actually evil? Ya don't say!