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.
Fridge Brilliance: The summoning ritual at the beginning of the game didn't fail. Another example ties into Fridge Horror; throughout the game, Rozalin displays a taste for meat, especially when undercooked. In the worst ending, Adell devours Taro and Hanako under the influence of Overlord Zenon. Rozalin must have eaten a lot of other demons during her days as an Eldritch Abomination...
Game Breaker: Dark Hero Days' DLC for Pleinair and Sapphire are free. They're both level 100. You can get them from the beginning. Pleinair joins you without a fight. End boss of the story is level 90. Do the math.
You can also recruit Sapphire at the beginning of the game by pounding on her with Pleinair while your normal characters act as bait as she will make a beeline for them due to the huge difference in level.
Thanks to how standard status effects work, the Sleep + Poison combo. Thieves can inflict both with a 100% hit rate in any enemy that is not a bonus boss. Since poison takes 20% of the unit's max HP, all it takes to kill any enemy in the game is to make them sleep, then poison, then wait five turns. The Kunoichi class goes beyond by being able to inflict the same statuses from four panels away in a cross-shaped area, hitting up to five enemies. A team of 10 Kunoichies with no equipment at Lv. 30 can break through Axel Mode without taking any damage, which means a lot when the enemies go all the way up to Lv. 250 there.
Due to what is perhaps a design oversight, you can capture a Lv. 9999 Nekomata as early as chapter 4. What keeps this from breaking the game in half is that you have to capture many Nekomatas on the way there and farm the item world for cash in order to heal those captured beasts. Once you do get a Lv9999 one, you never even have to use it on a single storyline map: the Item world bonus gauge prizes (high-ranked items and troves of EXP) are enough to steam roll the story and a good part of the postgame with your normal party.
Good Bad Bugs: A possible Overlook bug, You can make the White Dragon party like you by going into a bill as Senator Adell where you're supposed to disapprove by spamming an item at them. They won't take the item, but their Likeness towards you goes up, which will carry on other bills.
Fake Zenon crossed it when he destroyed the Snow Clan village just to take Rozalin and brainwash her.
Nightmare Fuel: The worst ending, in part because you do NOT expect this sort of thing out of this series. In summary, Adell kills his adopted siblings and eats them with realistic sound effects in the background.
The Sinner class. Not only did it have an uninspired design and bad aptitudes, but it replaced the Brawler classes from Disgaea: Hour of Darkness as the primary fist-users. The Geo Masters get similar hate for replacing the more popular Scout classes as the Geo Change users.
The Scrappy: An in-universe example. Just EXISTING is a crime for Tink. Considering how obnoxious he can be (especially when his red personality is dominant), though, it's hard to disagree with the game.
Woolseyism: Though many know him as Axel, it was actually a change made by NISA as his original name was Akutare. Some sources, including the credits at the end of the Japanese version of Disgaea 2, romanize it as Octalley.
Anything related to "Beauty Queen" Etna. Her title in the Japanese version is "Majin/Demon God/Demon Lord". A considerable amount of dialogue was added to make fun of the change, including the infamous line where she claims to have hacked her title.
Prism Rangers: "We are delivering justice... Even when no one asks us to." The second part of this phrase is US-exclusive.
Speaking of the Prism Rangers, in the original Japanese version Prism Orange speaks like a stereotypical Japanophile character in an anime, speaking Japanese with a distinct foreign accent. The American version inverts this by having him speaking Engrish phrases with an Asian accent.
"You have friends that care about you...and Etna." In fact, most of the jokes in the game were added in the English version.
The Kurtis clones were given a voice and speaking manner that parodies William Shatner.