Base-Breaking Character: Some people like Fuka for her spunkiness and how humorous and delusional she is, bringing many of the funnier moments in the game. However, some people hate her for the very same reasons, being delusional to the point that it comes off as stupid especially later in the game when she is still denying that she died when her killer told her she was killed. She also denies her own character development after you beat up her killer. Add to the fact she's part of the Spotlight-Stealing Squad causes a lot of grumbles in the fandom.
Designated Evil: The game hammers its Humans Are Bastards theme in again and again, and while it is true that the human world isn't in the best of shapes in the game, one can still get the feeling that this judgmental attitude comes across as extremely hypocritical when Demons routinely do far worse things to each other and never get called out on it. It becomes even worse when one finds out how the Netherworld and Celestia truly operate: humans are basically condemned to sit at the bottom of the cosmic food chain, terrorized by Demons so that they can get Fear Energy and Angels can get mankind's awe and belief in them. Overall, the whole situation can come across as the game condemning humans for daring to rebel against that situation (for example, by building anti-Demon weapons).
This system may actually be unique to 4, for it is never mentioned again in 5. Likely due to how badly the twist was received for being uncharacteristically mean-spirited and the fact that Disgaea 5 might be a prequel to the entire franchise.
Fridge Brilliance: The Ho Yay behaviour of Fenrich and his vitriolic hatred of Artina becomes such when you understand canine psychology. Canines tend to hate their master's new mate, perceiving her as an infringing threat on his territory and as taking said master from them.
Fridge Horror: Valvatorez's status as an Almighty Janitor takes a scary tone when it's discovered that Zetta (generally considered one of the most powerful entities in the Nipponverse alongside Zenon and Gig) of all people considers him a Worthy Opponent (post game fights starting with Disgaea 3 are canon). What kind of monster was Val when he was at his Tyrant state?
The flashback episodes confirm this: Tyrant Val is probably the biggest Game-Breaker and most powerful character in any Nippon game to date. However, it manages to avert the concept a bit too, since old Valsy is still a big goof and only treats being a Tyrant as a job, but he takes it just as seriously as he takes being a Prinny Instructor. He's just really, really strong.
Game-Breaker: Anyone who preorders the Premium Edition from Gamestop or NISA store gets access to Fallen Angel Flonne, who is an amazing archer and healer. And unlike the other DLC characters, you get her right at the beginning of the game!
Desco due to her powerful evilities (Increases her attack by 20% for every adjacent ally, gains 20% stats per third of total HP missing, and deals 50% more damage at the expense of 30% of normal mana gain), high stats, and multitude of long ranged area attacks that become even better after a monster fuses with her (Yog Sothoth hits a 3X5 area up to six spaces away, making it the biggest ranged area attack in the entire series). On top of that, she can be boosted even further by a Beast Trainer.
The Heart Cannon Evil Symbol, which allows anyone within its area of effect to be able to launch an extra attack on any target within its attack range if another character launches an attack on said target. If you plan your attacks right, this can lead to some ridiculous combos. And unlike counters, attacks activated through the Heart Cannon can trigger Team Attacks.
Chara World Duping got Nerfed, but has since been replaced with Shop Duping. It goes like this: Fill up the item bag and warehouse with junk items, level an item filled with subdued Innocents to ten, leave. The levelled item will be in the shop; trying it on will allow you to buy unlimited copies of said item.
Nippon Ichi seems to have deemed this method a little too overpowered, though, and it was later removed through a patch. Said patch can be deleted (at the cost of save data already present), but it's pre-patched in the Vita port.
In the Vita version, item duping was made infinitely easier thanks to bio suits getting a unique skill that doubles their stats but turns them into neutral units, rendering them susceptible to the puppy paw stick. Also the puppy paw stick has a higher steal rate too.
Emizel, due to the basic fact that he's essentially a prism skull with the stats of a main character that you can get WAY earlier than a regular prism skull.
Emizel can be made particularly deadly if you have downloaded Mao. His "Weakness Detector" Evility increases damage by 100% when exploiting an elemental weakness. Seeing as the entire point of a Prism Skull is exploiting elemental weakness, and Emizel already has an Evility that boosts magic damage by 50% against individual targets, he will basically do 250% damage against any single enemy who is weak to that element. Then teach him the S-Ranked Tera spells. Compare that to the Adell example below, who maxes out at 230% with only one element (Fire). Oh, and Emizel can do it at range. The Vita version makes him even more ridiculous by adding the SS-Ranked Peta spells.
Tyrant Valvatorez is shaping to be this. His natural ability increases all the stats of any given weapon by 50%, and his two other evilities are even stronger: one increases his stats by 20% for every ally around him in the HQ (meaning he gets a whopping 80% bonus) and the other turns every enemy killed by him into a ally for the rest of the battle. The former is so broken that it can't be passed to other characters in the PS3 version. Then again, you'd expect nothing less from the famed Tyrant.
Adell somehow manages to become even more broken than in Disgaea 3. In addition to Fair and Square, he now has a secondary evility that boosts fire attack damage by 20%, which of course stacks with both Fair and Square AND Vulkan Blaze, meaning that he has an S rank attack with a free 50% damage boost.
You can do even better with Adell, if you've bought Mao pass on his weakness detector Evility, it gives you a 100% damage boost if you attack with the element that the target is weak to and if the target is weak to fire, well, it's probably gonna be deader than dead after a Vulcan Blaze.
That's nothing compared to what a Magic Knight can do for him. Give Adell Elemental Force combined with casting Fire Boost, making Adell's Fire Apt 99%. Cast Anti-Fire on an enemy weak against fire like the level 1 slimes in a certain episode 5 X-Dimension map, causing them to be -99%. You already have a net 228% Boost. Combine with a maxed out bonus and you get a 868% boost, not taking into account the bonus for S ranked attacks. For added lulz, give him two giant Cu Siths that have been fused with dragons, causing a double tapped ATK bonus to elemental attacks based on movement combined with 2 ATK bonuses for all attacks fired at him (which included normal hits that did no damage).
The Trick Move evility, which increases your evasion bonus by 10% within the turn. Put this on your character, give him MOV boosting equips, and keep moving 10+ spaces and watch your character become untouchable. One of the few ways to get an edge on Baal in the rematch.
Good Bad Bugs: Like previous titles, the original version has an Item Duplication glitch; this time involving entering the Item World with a full inventory, and rebuying the intended item in a particular way. Not only was it very useful to clone rare or unique pieces of equipment to begin with, but it also cloned specialists, allowing for some truly obscene stat increases and other bonuses. It was too much of a Game-Breaker to be left in, and was patched out via an update and in the Vita port (though it can still be used in the original version if the update is deleted).
Hilarious in Hindsight: An episode of the Disgaea anime had the Disgaea trio disguised as Prinnies. However, Laharl just gets a Prinny head piece slapped on him due to a "miscalculation of the budget" according to Etna. Fast forward to Disgaea 4, where Fuka similarly gets a Prinny hat slapped on her instead of a full Prinny suit due to the inability to keep up with the rising Prinny population.
In Cross Edge, there is a gallery image depicting the results of Demitri'sMidnight Bliss on the cast. Prinny becomes a normal looking girl with a Prinny hat. This was about two years before Disgaea 4 brought us Fuka.
Magnificent Bastard: Fenrich can actually be this while manipulating everyone in the cast at some point.
Moe: Desco, as far as the Disgaea series goes. Not only is she physically adorable, looking like a cute little girl, but she's just so eager and determined to become a proper Final Boss that she becomes Adorkable.
Polished Port: The Vita port is the full game, plus all DLC, plus new content, and portable.
Replacement Scrappy: Averted with Valvatorez. He's been very well received by fans of the series.
Shocking Swerve: One of Disgaea 1's endings indicates that God is actually a decent guy. Come Disgaea 4, He's suddenly portrayed as an uncaring asshole. How this FaceHeel Turn came about isn't really explained or expanded upon beyond a vague explanation of "God is neutral" which is a shame because it's arguably a wham moment given where it appears.
The Vita version, however, gives clues that whatever unleashed Fear the Great wasn't "The Real McCoy", given some dialogue Valavatorez says after punching him in the face.
Spotlight-Stealing Squad: Fuka and Desco, post initial release. They got their own full-fledged DLC campaign as well as being the stars of the Vita-exclusive campaign as well. They also tend to get much more official artwork and merchandise.
That One Attack: Baal's Ultimate Force evility, which sends a giant sword straight to your character's head the moment they are summoned out of the base panel. If you're unprepared, he can annihilate your entire party before you can even issue a single command.
That One Sidequest: Opening the Land of Carnage, as usual. While possibly easier than getting there in D2, it is still one of the most annoying things to do in the game. Let's see... you need to build a Pirate Ship. Except not just any ship will do, you need a ship made of all the P Flonzor X parts, and of course the game itself doesn't tell you this. How do you get the parts? By capturing certain enemies on certain stages, then interrogating them to get a special treasure chest to spawn. Which stages do you need to do? Primarily X-Dimension Stages from Episodes 7 and 10. How do you unlock those stages? By going through the Item World and entering every mystery gate you see until you get lucky enough to find a Scalper, and lucky enough again that its the one that sells the Tickets for the latter half of the X-Dimension Stages. So you unlocked the stages where the target enemies are found? Good, now you just gotta make sure you've unlocked those enemies' classes for creation and that Valvatorez is higher level than them. Oh, and that you've got the Discipline Room in your HQ Map. So you managed to capture the enemies and got them to tell you where to find treasure? Guess what, you gotta reset if the chest didn't spawn on an X-Dimension map. And even then the piece you seek is an item that Randomly Drops from the chest. To add insult to injury, the chests can also give G Flonzor X or U Flonzor X parts instead... which unfortunately will not get you into LoC. So you successfully got all those parts? Too bad you're one short, and the last one is in a completely different place. After all it wouldn't be the LoC quest without ITEM WORLD PIRATE HUNTING. The good news, you're only after one specific pirate crew this time. The bad news? It's the Cat Pirates. Not only do they only appear on floors 91-99 (They start appearing from floor 21 onwards in the Vita version), but their appearance rate is RIDICULOUSLY low (average time spent resetting a floor to get them to appear has been reported at around 3-4 hours). OK, you FINALLY managed to put P Flonzor X together. Time to go to LoC at last! Oops, looks like Your Princess Is in Another Castle!! On top of all that, you also have to clear at least 25 X-Dimension maps. Thankfully, that's the easy part.
They Wasted a Perfectly Good Plot: Considering the impact Artina's death has on Nemo and by extension everyone in the game, it's rather disappointing that nothing on Nemo is elaborated on in the Vita exclusive scenario.
Uncanny Valley: The Prinny Factory game on the official website for Vita version tasks you with making Prinnies. The initial set of Prinnies seem innocent enough, most of them looking like your standard Prinny with little additions, but as you progress through the game, the types of Prinnies you can create get stranger and stranger. These include Real Prinny◊ and Prinny Man◊.
The Un-Twist: Turns out Vulcanus is Artina. The game holds the tension for all of five minutes of course, and half the characters make the connection before she even becomes a party member. The first time Valvatorez mentions the girl he made his promise to, it cuts to Vulcanus, doing everything short of outright saying they're the same person.