Demonic Spiders: The giant frogs appear out of nowhere, bother you during construction, have a hard to predict and hard to avoid One-Hit Kill attack, and can poison your men.
Fridge Brilliance: The rats don't seem to have much reason to act evil. But, if you notice, those same rats have been mimicking the king as he progresses through the adventure, with the king rat being the leader. That means they've watched him conquer the world, and are now trying to take the kingdom from Corobo.
Fridge Horror: The Giga Carpenters are robots. You have to send people in to make Giga Carpenters. You just forced your citizens to turn themselves into robots just so you can have some of the best carpenters in your army.
A common agreement amongst players is King Shishkebaboo of the Ripe Kingdom, which is a pinball boss fight where you have to use your troops to hit him across the board. And the areas that make him lose the most calories are pretty damn hard to aim for, not to mention you may have to hit a boulder blocking that part of the board several times to make it accessible in the first place. Did I also mention you have to time your hits just right so your troops don't end up hurt? You deserve a pat on the back if you win this fight the first time round.
TV Dinnah is pretty bad if you don't know Geography. This much is pretty obvious. But as the fight goes on, he starts to throw out fake hints (no less than 2 at the end), which wouldn't be bad on its own if you didn't have a strict time limit. How bad? You have less than 20 seconds to guess the last one correctly if you actually wait for the real hint. Worse yet, if you only have one digger left by the end of the fight, there's no way you'll be able to stop the clockwithout guessing and hoping you pick the right place. And there's a lot of choices you can pick incorrectly.
That One Sidequest: Gathering tunes for Princess Kokomo. You have to gather tunes by wandering around your sprawling kingdom and talking to random villagers with notes above their heads. Except villagers only give tunes between 10pm and midnight on the games clock (about a 10 minute window). Oh, and at no point does the game tell you any of this.
"Weird Al" Effect Younger players won't recognize the songs from classic Warner Bros. and MGM cartoons which are the origins of Classical music in comedy, which was to prove that Classical music can be used for anything.