Anticlimax Boss: The master of the Nighthawks is a major villain, and perhaps the third most important enemy in the game story-wise. However, he fights you solo, against three PCs, one of whom is a mage who can keep him permanently stunlocked with Despair Thy Eyes or Fetters of Rime. He simply has no chance. The oddest part is that someone took great care to make him a viable boss - he's one of the few enemies in the first chapters to carry heavily enchanted weapons, and is immune to most major insta-death (or equivalent) spells. He just wasn't supplied with the few flunkies necessary to make him actually somewhat challenging.
Ensemble Dark Horse: Gorath. Lots of fans and critics alike have called for more books with him. Getting this kind of reaction is no mean feat, especially when Jimmy the Hand is also on the stage.
Some of the final books in the cycle have introduced Arkan, who is Gorath's son, and briefly bring back Liallan as well. Your mileage may vary on whether they (and the story potential the plot threads left loose after BaK) are used well, however...
First Installment Wins: Sierra's management treated the game as just a marketing cash-in, and broke up the team that make it before they realized that they had a hit on their hands. As a result, while they later attempted to make several followups, none of them were able to compare to the original.
Fridge Logic: At the start of Chapter IV, just how is Owyn able to understand what Delekhan is saying (presumably in moredhel) to Gorath? However, given that his resulting lapse of trust in Gorath doesn't actually affect anything, this isn't such a big deal.
In the novelisation, however, the language barrier problem is ignored on far more numerous and significant occasions. Unless you're willing to assume that Gorath would begin a private conversation with his estranged wife in anything other than their native language...
Game-Breaker: The spell Skin Of The Dragon, which can be found fairly early on during chapter one, grants absolute immunity to any damage for a number of rounds, and can be recast again and again during a combat. Even a character that has been poisoned will take no damage while experiencing the effects of this spell. It works against every foe in the game, and even the final battle can be easily won by casting this spell on the party members and then whacking the Big Bad and his Mooks with sticks.
In two chapters your spellcaster has no access to this spell, and those two chapters are easily the two most difficult in the game. However, with a little foresight you can get him this spell by leaving its scroll in a chest or something and breeze on through the rest of the game.
You can also have Owyn practice lute playing to absurd degree that becomes a nice source of income at higher levels.
There are actually a ton of ways to make a lot of money quickly. For example, at the beginning of Chapter III a guild war has broken out in Romney, causing the prices in the shop to increase sixfold until a subquest is completed. This also means that the shopkeeper will buy stuff from the party at six times the normal price, and with other normal-priced shops only a half-day's travel to both the north and south, it's quite easy to amass a lot of money here. There are several other places to also earn a lot of money, many of which are detailed here, about halfway down the page.
The "Evil Seek" spell. Chain Lightning-type spell that wipes out any attacking party in two casts.
If the first target is an ogre, the damage to any enemy is doubled. Guess who you fight right after finding this spell?
Steelfiring the Guarda Revanche, which never decays, will last forever, so every blow Gorath strikes with it is strong enough to kill virtually any enemy. If you want to really get cheeky, you can then go and bless the blade.
This actually can become a Game-Breaking Bug, as just like the example on the main page, it's possible to start doing negative damage to the enemy if Gorath's strength gets high enough (which it easily can do before the end of the game, even without the well mentioned on the main page). In fact, in the original diskette version the blade was blessed when you found it, but due to this bug the designers took the blessing off. Kind of a stopgap solution, since it's easy enough to get it blessed anyway.
Good Bad Bugs: In combat you can do anything and then rest on the same turn, if you press "R" quickly enough. Or you can rest twice. This allows to recover quicker, and to waste no stamina on swings and "Despair thy Eyes" spells.
That One Attack: The spell "Grief of 1000 Nights" freezes a character for a number of rounds. This is nice when used by one of your mages; however, if enemy mages use it on your party it is far deadlier, as when all of your characters are frozen the game treats it as a Game Over, even if their hit points are still high enough to survive more attacks before the spell wears off. It gets worse in combats with multiple enemy mages, especially if your party screws up a surprise attack and goes last, as sometimes you'll get frozen up so quickly you may not even get a turn.
Made even worse if you don't find Nalar's Rib, as both the stone itself and the note with a clue to its existence are quite easy to miss.
Tier-Induced Scrappy: Patrus effectively acts as a replacement of Owyn as the designated mage in his team. However, compared to Owyn he tends to have worse stats and, more importantly, a poor selection of available spells when he is first introduced; including lacking a means of ever getting the Game-Breaker spells without actively planning to leave a copy for him. This makes Patrus feel rather inferior compared to Owyn.