Awesome Art: Though criticized at the time for lacking polygonal models in reviews, the extremely detailed sprites and character animations have barely aged the game.
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.
Good Bad Bugs: There is a Game Breaker glitch in the PSP release of the game concerning the items Foresight and Tainted Staff. Their Item Worlds are ridiculously glitched, full of Laharl clones starting at levels in the several hundreds and climbing even higher. However, the items also come with support specialists, including Brokers, Armsmasters, Statisticians, and Managers, with levels in the five-digit range and they're already tamed, so you can buy one of these items and swap the specialist to another item instantly. The normal stat-increasing specialists also have levels in the hundreds, which double when tamed as usual. While even a party capable of defeating Baal will be promptly crushed by the armies of Laharl clones, if you can get in, manage to subdue one of these specialists, and get out safely, you can get stat boosting specialists with levels in the thousands and make your party very overpowered. Additionally, the Laharl clones equip end-game items with Ranks in the high 30s. Sure, your Thief might only have a 1% chance to steal them, but when that one in a hundred chance pays off, it's gonna be worth it to nab that Legendary Accelerator, Cosmic Blade, etc.
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 Earth! Gordon's bombastic personality has endeared him to many a fan.
Mexicans Love Speedy Gonzales: Captain Gordon is a parody of 50's and 60's-style American sci-fi action heroes. He is also hugely popular in America, much more so than Japan.
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.
Narm Charm: Exclusive to Etna Mode in the remake, Justice Flonne's So Bad, It's Good Sentai cosplay is just a helmet, but she's so enthusiastic in her persona that you can't help but find it adorable and more than a little awesome. Helps she's your rival in that mode, and eventually gets a bit challenging near the end.
Nightmare Fuel: The angel enemies emotionless attitude makes them surprisingly creepy despite their appearance.
One True Threesome: Laharl/Etna/Flonne, one of the most iconic instances in an RPG. They play off of each other so well that fans find it an obvious solution.
Player Punch: Flonne sacrificing herself to protect Laharl and her friends for her sins as a fallen angel. She's not even regretful about it. Regardless if you've gotten any of the good endings after, the scene itself is still powerful.
Polished Port: The PC release has been stated to be "not just a port", with updated interface and map textures that look good at 1080p.
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.
Scrappy Weapon: Bow weapons. Their damage being based on ATK and HIT meant most players found getting a good bow user was too much of a pain. Not helped that guns had much greater range than them and more powerful special attacks. The only real reason you'll ever use a Bow is to unlock the Archer class (in turn to unlock the Celestial Host) and so your Clerics have an offensive weapon (assuming you use unmixed Clerics at all), otherwise they're useless.
Thieves, on account of sucking hard in pretty much every way. Even the highest rank of Thief, the Space Pirate, has only a C-rank with Guns, 60% and 70% aptitude with all stats except Speed and Hit, and has lower base stats than even first-tier jobs in other classes. However, Thieves are the only class that can throw with a range of 6, they're the only ones that can steal reliably, and are needed to unlock the Majin, so you pretty much have to use them if you want to get the most out of post-main quest gameplay.
Despite being a beloved class due to its cute design, its hard to justify using the Archer class. They have rather pathetic stats and their highest ranked weapon is the bow, which is unfortunately the game's Scrappy Weapon, with no other weapon reaching beyond rank D. The only reason to have an Archer is to unlock the much better Celestial Host class.
Hoggmeiser and Maderas, two monster characters that join during the story, are actually weaker than their generic monster counterparts (Nether Noble and Vampire respectively). They're decently strong when they're acquired, but are quickly outclassed by custom characters.
Monster classes, in general. The inability to lift and throw, lack of a Weapon Mastery to increase their damage potential, and general lack of versatility make them a hard choice when compared to the humanoid classes. Luckily, later Disgaea games introduce various mechanics that allow the monster classes to be just as useful as the humanoids.
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.