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.
A more comedic example in the first game with what exactly Laharl's embarrassing picture depicts. Many fans are convinced he was cross-dressing.
Evil Is Cool: Many characters, both heroes and villains, which is inevitable given the franchise is set in (an admittedly comical) take on a World of Badass. Laharl is notable for attempting to achieve this status In-Universe.
Evil Academy is an inversion, however. You're a badass rebel if you break the rules, which a demon does by becoming a goody-two-shoes.
The actual villains in the games mostly avert this, either by being impossible to root for due to being actually evil unlike most demons (namely Vulcanus, the fake Overlord Zenon, and Super Hero Aurum), or by not being evil at all (namely Seraph Lamington and Xenolith).
Fan-Preferred Couple: All things considered, Laharl/Flonne is probably the most popular flagship of the franchise.
First Installment Wins: Many fans say that the original Disgaea has the most iconic storyline and characters out of all of them. The fact that the Disgaea trio are always the top three on almost every Nippon Ichi poll is proof of that. It should be noted, though, that this does not apply to gameplay, which just keeps getting better and better as the series goes on, whereas the original installment is widely considered to be the most bare-bones in terms of gameplay mechanics.
The Developers seem to have acknowledge this trope as well; every sequel in the series have featured the iconic trio in one wayor another.
Well, in Disgaea 2 (at least in Dark Hero Days) all you need to do to unlock the Majin class is beat the game once and pass a bill to create the class in your New Game+. However, that particular class from Disgaea 2 onwards has been nerfed. Despite possessing superior aptitudes and stats, their pathetic MOV makes them less useful than other classes.
However, the Argus, the red coloured galactic demon, is available MUCH earlier. A level 300ish Argus is damn near impregnable to anything and anyone up to level oh, 900, with the proper defensive build. You can unlock a lot of bonus levels early by taking an Argus or two with you, and either pumping them with attack (if you want to soften the target(s) up with melee) or defense (for making the enemy waste all their MP before you officially come out to play.) You can get access to Prier by level 180 thanks to the Argus. Not that she can be beaten by such puny levels... there's quite an insane difficulty jump between the extra characters and their gate guards, of course. Still, the galactic demons, even the weakest, have such ridiculous defensive stats that you can beat the entire game using them as Schmuck Bait towards anyone a higher level than you. Their counter rate is also pretty nice. The only con against them is, being a monster type, they will likely fall behind in damage capability, especially since that's one of their lower-end stats.
Special mention goes to Adell in the third and fourth games' DLC, more specifically Vulkan Blaze. An S rank fire move (the strongest elemental technique in the game) that targets one panel. Doesn't sound that broken until you realize that Adell has an evility that inflicts 30% more damage in one on one combat, which essentially gives said S ranked attack a free 30% boost. In addition, many of the boss fights have enemies in which fire is the best option.
Pleinair is also very broken in both the first game and the third game. Given the long mission you have to do to get her in the third game, she's worth the trouble.
In the Updated Re-release of the second game, Plenair is a free DLC character. She starts at level 100, is available at the very beginning of the game, and joins without a fight. So long, any challenge for 90% of the main story...
Good Bad Bugs: In the PSP port of the game, the Foresight and Tainted Staff items are glitched. They often come with specialists subdued like Brokers and Armsmasters, and when the Foresight is equipped it lowers Jump to negative digits, but when a second is equipped their effects cancel out and Jump is higher than normal. Additionally, the Item Worlds in each consist of the highest-level enemies in the game starting in the hundreds and reaching into the thousands, making it an excellent place to level grind and steal good items. The catch is what those level-thousand enemies are — Laharl clones — but if you can take them...
Hilarious in Hindsight: All the Prism Rangers battles become outright hilarious when you realize that in each game, they've been fighting a former Mighty Morphin' Power Rangers cast member. In the first game, as noted above, they fight Rita Repulsa in order to claim the throne. In the second game, they fight Scorpina in a tournament in order to face Overlord Zenon. In the third game, Prism Red comes face to face with the former Black Ranger. Then, it all comes full circle when said former Black Ranger returns as the Blue Prism Ranger in the fourth game's Fuka and Desco Show.
Disgaea 4: A Promise Unforgotten has Petta, who is an odd blend of the above 2how? The original game was dubbed and brought overseas but the remake where she is introduced was not and she was DLC like Kogure but Her cameo spoiled a plot point from Makai Kingdom's remake like how Prier's cameo spoiled the Non-Standard Game Over from her game (and maybe canonized it in the process)
Porting Disaster: Some fans consider Disgaea DS to be this. Pixelated graphics, removal of most voice acting, and sluggish performance compared to the PS2 and PSP versions. Of course, in trying to fit a PS2 game onto the DS, some may say this was inevitable.
Replacement Scrappy: There is a sizable amount of fans who are very vocal about how they prefer Laharl over nearly all of the sequels' protagonists.
Somewhat averted with Valvatorez in Disgaea 4, as he's been generally well-received by fans. These days arguments about who the best protagonist is usually come down to a tie between him and Laharl.
Valvatorez is also a victim of this, in regards to both Fenrich and Artina.
Sidetracked by the Gold Saucer: It's surprisingly easy to get addicted into gaming the Dark Assembly. Perhaps the easiest way to to farm for items in the item worlds, which can power level your equipment and your characters, and gives you the mana to try even more fun bills... wait, what am I forgetting? Oh, right, the plot...
Unpopular Popular Character: Asagi is treated as the biggest loser in the entire series, earning pity from almost everyone who comes across her. She has a legion of fans among the playerbase.
Laharl to a lesser extent. Polls have shown him to be the most popular character in the series, but in most of his appearances he suffers from Dude, Where's My Respect?.
The Prinnies are of the series main icons, and get almost no respect in universe, which admittedly is part of why fans like them.
The Woobie or the Butt Monkey: Some fans find Asagi's suffering hilarious. Others want to wrap her in a warm blanket and assure her that she'll get a game of her very own someday.
Extra woobie points in that, if she DOES get her own game, her entire reason for being in these situations disappears. She is officially doomed to suffer forever, or achieve happiness once, and then never appear again.