Sealed Evil in a Can: Unusual in that the Evil Sealed Itself In The Can. The King of Shadows was injured badly enough by Nolaloth's attack that it retreated to the Astral Plane for roughly a thousand years.
Sequence Breaking: The game intend you to have killed the cleric in the graveyard before going to Highcliff. There are also others here and there, including in the form of Script Breaking.
AI is much, much smarter. It's still overall bad, but you have limited control over some of its decision making process, it doesn't randomly stop moving or randomly run off to the other side of the map, and, if all else fails, you can manually take control of everyone in your party.
Many of the spell lists and classes were closer to their pen and paper counterparts than in the first game, removing some rather arbitrary restrictions. Druids probably benefited most since they didn't even get access to most of their spells, only counting the ones included in the game in NWN.
Larger party size meant that you could have a dedicated skill monkey, arcane caster and divine caster in the group. This results in a colossal increase in effectiveness of all party members well beyond numbers alone.
Better leveling curve. In NWN, early leveling was fairly slow, but in NWN you leveled fairly quickly initially and then it petered off. This is important since a lot of builds are not very effective until level 10 (give or take a couple levels). This meant in NWN, you spent a large amount of game time with characters with under-developed class abilities (especially hybrids) in comparison to NWN2 where you got past those rocky levels fairly quickly.
Some of this is the result of the sequel being based on 3.5 D&D rules where the first game was based on 3.0 . The pen and paper game was rebalanced to address similar issues.
Sliding Scale of Law Enforcement: Neverwinter's Watch runs the gamut from honest and just fighters of corruption (mainly the upper echelon, including Captain Cormick) to guards who want to fight corruption and the Shadow Thieves but are too scared, to guards who are outright on the Shadow Thieves' payroll.
Tarmas:(referring to a piece of doggerel he had to put together for the Harvest Festival) Count yourself fortunate for that. I once shared a wagon with a kobold bard. He rhymed "sadder" with "bladder" and sang fondly of his navel.
Story Branching: The game offers you a choice when you get to Neverwinter. You can either side with the Watch or with the Shadow Thieves. Either choice eventually gets you into the nobles' section of the city to advance the plot, and gives you different defense options during your murder trial in Act II.
Stripperiffic: Parodied. Your more human looking female party members can never be stripped below rather bulky clothing and even the tavern dancers wear long full frilly dresses. But you get one female teammate late in the game who is dressed in something resembling the Princess Leia slave outfit, and she's a githzerai, meaning she looks like a humanoid salamander.
Suicidal Overconfidence: Of course! In one notable late-game encounter, you run into a group of bandits who intend to kill you for your magic items and the Luskan bounty on your head. You can remind them that you've already left thousands of bodies behind you. If you pass your Intimidate roll, some of them run off, while the rest charge into pointy death at the Knight Captain's hands.
Grishnak: You're a strange lookin' group. What'dya want? Knight-Captain: ... says the half-orc pirate.
Useless Useful Stealth: You might be a stealthy Rogue and take all the feats and all the invisibility potions you want, but you cannot slip through forced dialogue triggers. Which is problematic because after these, a fight tends to break out, and now your party members have teleported in the middle of it. You might remain undetected afterwards, as illogic as that sounds. Also, every enemy in the area that has ever seen you, even if just once, will do nothing but follow you and prance around your general location until they roll a lucky 20 on their Spot check, or you decloak. Roughly a third of the enemies in the game are undead, which makes sneak attacks moot. And in any case there are so many enemies in every encounter it makes almost no difference to single out one of them beforehand.
In Mask of the Betrayer, literally battling for your soul inside your head on the Fugue Plane.
In Storm of Zehir, the comparatively much shorter Temple of the World Serpent.
Villain-Beating Artifact: The Silver Sword of Gith is the only weapon capable of harming the King of Shadows. Unfortunately, it was broken into several shards the last time it was used, and your character isn't the only person interested in collecting them.
Vision Quest: Khelgar has one to become a monk. Which is a possible subversion. While there's nothing stopping the player from going through the motions, Khelgar's stats are suboptimal for a monk. He's one of the best tanks of the game as a fighter but as a monk he loses most of his power. Also, a late-game ancestral Ironfist weapon that he should be able to equip can no longer be done, due to the class change.
Warp Whistle: Exit points on exterior maps take you to a Point-and-Click Map in the OC and Mask of the Betrayer. In Storm of Zehir the exit took you to an overworld map instead.
We Are Struggling Together: Some of the antagonists, such as the Githyanki and Ammon Jerro want the King of Shadows defeated as badly as you do; they're just not interested in working alongside you or anyone else.
We Buy Anything: Except items worth 0 gold, which you're forced to drop. Also, vendors have a limited amount of cash on hand, and they stop buying when it runs out.
The sealed ruins just outside of West Harbor for the final bit of the ritual. Perhaps more literally, the final dungeon, which is implied to be directly beneath West Harbor.
Plus the dream sequence that houses the final boss fight in Mask of the Betrayer takes place in West Harbor.
Wild Card: Bishop. In his first interactions with the player, he makes it perfectly clear that no matter what, he is always on the winning side, loyalties and morals be damned. This comes back to bite you when the King of Shadows gains the upper hand near the end.
You can optionally make him desert the King of Shadows, but you can't make him fight alongside you again. Unless you play an Evil PC and choose to side with the King of Shadows yourself.
Woobie, Destroyer of Worlds: The King of Shadows started out as one of the greatest heroes of ancient Illefarn. He voluntarily underwent a horrifically painful ritual that transformed him into a construct of pure magic so as to make an effective deterrent to Netheril. When the Netherese wizard Karsus tried to usurp Mystryl's place as god of magic, the Weave was interrupted and the Guardian faced destruction. So he drew power from the Shadow Weave to continue his vigil and became the King of Shadows.
This is subverted in that the Translated Foreign Word isn't translated right away by the people who call you by it. The first attempt to translate it doesn't go so well, though it does count as a Crowning Moment of Funny for Grobnar Gnomehands:
Grobnar: Well, it's not Gnomish, Elvish, Dwarvish, Orcish, Goblin, or Draconic — well, unless the 'k' is silent, but that would make it "gizzard stone" or the equivalent.
When you finally meet somebody who speaks Gith, it turns out that kalach-cha directly translates as "shard-bearer". With full nuance applied, it means "one who stole a silver sword and destroyed it to hide their crime". Which ironically makes the term more applicable to Ammon Jerro rather than the Player Character.
The best part? Such an event has never happened before and they created the Translated Foreign Word specifically to describe you.
Well, there was a broken "standard" Silver Sword in Baldur's Gate II. Wouldn't that have made Saemon and CHARNAME both "kalach-cha"?