Eye of the Beholder/YMMV

• Anticlimax Boss: The Big Bads of both games are relatively easy to handle because of level design. The first game's big bad becomes much more climatic if you defeat him properly with the quest item Wand of Silvias.
• Demonic Spiders:
  • In the first game, applied literally with the Giant Spiders. You meet with them in a part of the dungeon where you're unable to backtrack and must find the way out, while too low-level to have any spell that durably neutralize their venom, and thus dependant on the few counterpoison potions you can find.
  • 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.
• Fanon Discontinuity: The third game is generally considered the weakest of the trilogy, and as such, many would rather pretend it ended with the second one.
• Game Breaker:
  • The Stoneskin spell in Eye of the Beholder I. For details, look here. Not surprisingly, it completely disappeared in Eye of the Beholder II, even for a party coming from the previous game, an exception to the Old Save Bonus.
  • In Eye of the Beholder II: The 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 Advanced Dungeons & Dragons rules.
• Good Bad Bugs: The Aid spell can raise a character's hit points total above the max, but only temporarily (unless right before a fight, it's unlikely to be of much use). However, damage from a fall mucks up the calculation, and you can end up with more hit points than before for a while, even above your maximum, including after the expiration of the Aid spell.
• 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.