Author's Saving Throw: Some fans took issue in the first game about how Laharl's Mom's sacrifice to save Laharl was interpreted as a sin, leading to her becoming a Prinny. The issue rectified a bit by revealing that she always had the choice of becoming an angel instead. She chose Prinnyhood so that Sicily, who she was pregnant with at the time, would live.
Awesome Music: Dramatic Devil Story is fittingly epic for the battle with Xenolith.
Broken Base: The return of the Weapon Mastery System. As mentioned on the main YMMV page, some fans were glad to see it go when Disgaea 3 came out while others weren't as enthusiastic. While being able to tweak the Weapon Mastery rate with the Cheat Shop and some other changes help alleviate some of the backlash, the base is still divided on which of the two methods of gaining skills was better.
The Monster Mounting replacing Magichange. Those who prefer Monster Mounting like that it allows monsters to ferry the humanoid characters, act as a mobile meat shield for weaker characters early in the game, and is not limited by a turn limit. Those who prefer Magichange don't like that humanoids are limited to magic skills while mounting, that the overall uniqueness and stat boosts from Magichange outweigh the turn limit, and that while mounting looks perfectly fine for larger monsters like dragons, it ends up looking silly on the more humanoid looking monster-types like Raspberyl and Metallica.
The plot itself. While many were pleased to see the main trio return to their pre-Flanderization characterizations from the first game and enjoyed the newly introduced cast members, just as many found the Long Lost Sibling angle to be cliched, were put off by Sicily permanently joining the trio, or simply missed the Defenders of Earth.
Character Rerailment: The main cast has been hit hard by Flanderization in their guest appearances over the years since the first game, which at first glance seems to have stuck as of the beginning of this game. In truth, it's more a case of Jerkass Fašade and Vitriolic Best Buds than anything, and the character development did stick (and in fact gets expounded on), it's just not obvious from their day-to-day interactions.
Demonic Spiders: Anything that can inflict Poison in your characters, mostly because they are always accompanied with Mothmen on the map. By simply being present on the map, the Mothmen cause the Poison skills to hit much more often, last considerably longer and deal MUCH more damage per turn. In fact, if not cured, being poisoned while a Mothman is around will kill your character in three turns even at full health, even if you have hundreds of millions of hit points.
Designated Evil: The spreading of the Yuie flowers for most of the game. Everyone tries to stop it because it will turn the Netherworld into another Celestia, even though the worst thing that they do is cause minor changes to demons which are only temporary. It's not until it turns out that Xenolith is having angels drained of their life, essentially killing them to do it that it's revealed to actually be bad thing.
Ensemble Darkhorse: Laharl's female form became an immediate hit with fans as soon as it was revealed. Likely intentional, as the other characters comment on how they like him much more as a girl, and it's much, much easier to turn him from a boy into a girl in the Dark Assembly than vice-versa.
Fridge Logic: According to the artbook, Sicily is 937; nearly 400 years younger than Laharl. Are we seriously expected to believe their human mother lived for that long after she had Laharl?
Salvatore because her evility. It performs one of several randomly selected buffs to adjacent allies, but when the options are increasing movement by 10, range of attacks and spells by 9, jump by 99, doubling damage dealt, and cutting damage taken down to 10 percent of normal, you can't really complain.
Pram's evility includes poison (that always hits), a hit on a random enemy, a free 50% INT boost for all allies, a 100% Attack boost for herself and 50% evasion for all units.
Human Priere brings with her the Purify Power evility, which, when damaging a unit, spreads that damage to adjacent unit and all other after it with decreased damaged as it spreads further out. With the right setup, a character can clear an entire Item World floor. The best part is that the character with Purify Power gets all gains (EXP, Mana, HL, and even Land of Carnage stats) from every unit killed this way. See for yourself.
Memetic Mutation: Laharl is wearing pants! explanation Laharl's shorts have become such an iconic part of his character design (alongside his antenna hair and scarf) that when his new character design showed that he traded them for pants, many fans became far more fixated on his new pants than on anything else.
The events of Chapter 2, how Laharl meets and befriends Sicily is eerily similar to how Laharl meets and befriends Flonne in Chapter 2 of Hour of Darkness, down to her summoning Zombies and Dragons to fight the protagonists.
That One Achievement: Special Voyeur requires that you see every single special attack animation. This normally would have been easy, if a little time consuming, were it not for the fact that it includes the 3 unique skills of Xenolith's first phase and Darkdeath Evilman. You will only ever fight either boss once per cycle, meaning that if they go down before you get to see their skills, you have to start a new cycle just to see them. It can still be a pain in the ass trying to view them even if you're trying to, as there's no guarantee that they'll use all three in one fight, though that's mitigated by the fact that getting a Game Over in this game will simply send you back to the castle.
That One Boss: So, thought Baal in Disgaea 4 was too easy? Apparently, so did Nippon Ichi, because they took that and made him even harder. He can only be fought in Raesetsu Mode, so his stats are going to be stupidly high (five-hundred million at the very least), his items can't be stolen, he never loses his stationary bonus so he will hit harder the longer the fight drags outnote normally, stationary bonus adds extra power each time a unit attacks without moving, but the Super Time Engine Baal has equipped negates that penalty, has Ultimate Force from Disgaea 4 (the evility in which one of his swords drops on your character the moment it's summoned from the base panel), and a variation of Pringer X's Super Skill Solved evility (specials from a single character cannot be used more than once). Bring him down, and it turns out he has a new form and with it a new evility and will do this five more times. New evilities include inflicting all status effects, increased evasion, summoning mini-Baals, automatic damage at the end of turn, and base panel destruction. Good luck.
The second battle against Xenolith is nigh universally agreed upon to be the hardest non-Bonus Boss in the entire series. He's the weakest main final boss base stat wise, but his evility more than makes up for it. It siphons at most 30% of the stats of any character within 10 spaces of him. That's right: he renders grinding, a simple and powerful strategy, completely and utterly useless. A proper plan and intelligent team composition are actually required for once, which can throw off anybody who just powers their way through the games.
That One Level: The battle against Barbara on Chapter 8. You have 5 turns to kill Barbara, three Baciels and four Sea Angels in a stage almost entirely covered by Recovery 20% (everyone recovers 20% of their max HP at the end of each turn), Defense + 50% (halved damage), Ally Lonely (if one of your characters is standing at a red panel, none of your other characters may step in any other red panel in the map) and No Lifting. This is the point where players raising a unbalanced party will have to stop everything and grind one character to be a One-Man Army or grind a balanced party by replacing and creating new, better planned characters.
True Neutral: Barbara refuses to do anything without being given an order, and doesn't appear to have any concept of morality.
Uncanny Valley: While likely intentional given how it's mentioned the work on them was done hastily, Laharl's statues look surprisingly creepy.
What an Idiot: Barbara doesn't try to win her first fight with Laharl because she wasn't ordered to and in one of the bad endings, doesn't run for her life when a black hole forms because she wasn't ordered to.