These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crazy Awesome: It all began here. One of the most glorious examples is Laharl's ultimate skill, Meteor Impact. He summons a giant meteor to fire at enemies, then backflips onto it and rides it into them with his trademark Evil Laugh.
Demonic Spiders: In the main story, EDF soldiers because their high levels, but they're put to shame by the Celestial Hosts that comprise all the enemies in the final episode. Outside of Majins, mentioned bellow, they are easily the best class in the game, excelling at physical combat and magic.
Mid-Boss gained popularity due to his sheer hilarity, even though he has roles beyond that.
"Funny Aneurysm" Moment: If you want to get a chuckle out of Laharl's fear of well-endowed women at all, don't ever, ever read about the novels. 100 years of physical and emotional abuse is not exactly laughing material.
Game Breaker: Majins. They have 110% aptitude in all stats, 120% at higher tiers, S-rank with all weapons, and their first tier form has stat growth rivaling the highest tiers of other classes. They take a long time to unlock, requiring five specific jobs at Level 200 and all in your party at the same time, and the cost to transmute a character into one is huge. But once you get them, the game is ridiculously easy.
Growing the Beard: Creator wise, this is where Nippon Ichi started using the sort of parody style humor and pop-culture references their later games would be known for and saying their games were set in a shared universe.
Memetic Molester: Mid-Boss has had this status for quite a while, due to his rather stalkerish behavior, especially prominent in the manga, in which the other characters flat-out accuse him of being a pedophile.
Memetic Mutation: Captain Gordon, Defender of—Oh, screw it. If you don't get it by now...
Moral Event Horizon: Laharl crosses this in Flonne's mind if he opts to kill Hoggmeiser in Chapter 3 (which opens up if you've got at least 50 ally kills up to that point). She decides at that point that demons are evil after all.
Nightmare Fuel: The angel enemies emotionless attitude makes them surprisingly creepy despite their appearance.
Re Watch Bonus: Start the game over after learning that Mid-Boss was working with Lamington, re-watching the scenes where Lamington is speaking with his unseen collaborator, one can tell that can see Foreshadowing to it being Mid-Boss.
Woolseyism: In the Japanese text, King Krichevskoy apparently died from choking on a Dark Manju note Dumpling of the Damned in the anime. In the US version, the food was changed to Black Pretzels, causing a Hilarious in Hindsight from when George W. Bush briefly lost consciousness after choking on a pretzel.