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.
Crowning Moment of Funny: The two Drama CD's have some amusing events, such as Laharl trying to recruit Ash into a band which he names "The Laharls" (and Ash's deadpan reaction of "Not happening..."), and, upon trying to recruit Castile into a Pettanko Quintet alongside Marona, Myao, Flonne and Etna, Etna herself breaking down into tears over the fact that Castille has more chest than she does.
Crowning Moment of Heartwarming: If you didn't shed a tear of joy the first time Marona met Castille and finally, finally made a friend after being consistently treated like garbage by pretty much everyone else in the world except Ash...
Game Breaker: Not to be confused with the BGM Game Breaker. The Failure title modifies anything it's attached to by reducing its stats by 90% and XP by 100%. When attached to a dungeon, everything in that dungeon gets that dungeon's title, making everything in it extremely weak and worth little XP. Due to an oversight, Phantom XP gain from being confined to items is not affected by this. Creating a high level dungeon, throwing the failure title on it, and diving in for a few levels is an absurdly fast way to gain hundreds of levels without any work (except for Marona, who gains a set % of a level each battle).
To a somewhat lesser extent, the ability Quick Attack. It guarantees the character will get an attack, which can be very good for high ATK but low SPD weapons, like axes. On the downside, only two classes come with the ability naturally, and it costs an abnormally high amount of mana to transfer. But if the skill is fused to a slime before transfered onto the character in question, the cost is laughable.
Trolleys. Particularly, high-level trolleys picked up through failure dungeon grinding. A SPD-boosting item with SPD-based attacks? Yes. Hell, yes. Now watch Marona club to death everything on the field before anyone else can get a single turn. Who needs Phantoms?
Generic Doomsday Villain: Sulphur. He isn't so much a character as he is a plot device need to move the story along. He has no backstory, no explained motivation, no personality, and doesn't even talk.
Tear Jerker: So, so many places in the game. It's hard to even begin listing them...
To put it into perspective: Your parents are dead. Everyone calls you "The Possessed One" because your guardian hangs around you. Said guardian is trapped in limbo, unable to feel anything at all. It isn't until the last few chapters or so everyone starts treating you like an actual human being.
That One Level: Any stage with few items on the field. Fewer items = less confinable Phantoms, which means relying on one overpowered Phantom rather than a team of roughly equal Phantoms. Either that, or use Marona for anything other than healing purposes.