Eye of the Beholder/YMMV


 * Demonic Spiders
 * In the first game, applied literally with the Giant Spiders.
 * In both games, thri-kreen (mantis warriors). They're incredibly fast, tough, and have a paralysing bite that makes it pretty sure one of your meat shields will be incapacitated before having even a chance to strike. So much that fighting them one-by-one is actually a less favorable option than facing a swarm of them -- then at least you can spam offensive spells more efficiently.
 * Game Breaker
 * The Stoneskin spell in EotB1. For details, look here. Not surprisingly, it completely disappeared in EotB2, even for a party coming from the previous game, an exception to the Old Save Bonus.
 * In Legend of Darkmoon, though they are both high-level spells, the combo "Wall of Force + Cone of Cold" can be quite effective. To clarify: a Wall of Force stops all the monsters and every attack spells -- except Code of Cold which can be used through it, and cover a wide area to boot. May be a Good Bad Bugs since that shouldn't be possible by AD&D rules.
 * Porting Disaster: Apparently, a Game Boy Advance cartridge can't hold as much information as a DOS game. Who knew? Anyways, the GBA remake of the first game left out all but the four basic character classes and much of the gameplay. Instead of the first-person combat of the original, it uses an isometric system like the gold box games.
 * Scrappy Mechanic: The tiles that spin the party around.