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.
Disappointing Last Level - In DQM 2, as the main worlds grew increasingly larger and more elaborate, with a thorough story each and a wide variety of inhabitant monsters, the final main world Limbo is a very anti-climatic end. Limbo consists entirely of a small island about six screens large, with just a few random encounters and nothing but the entrance to the final boss' castle. Then inside that castle, you just fight your rival one final time, then progress through the castle, which just consists of a room about eight screens large with no obstacles but the same few random encounters you fought outside in Limbo, and then the final boss' throne room, where you just fight the final boss. If you came into Limbo sufficiently strong enough to beat the final boss, you could clear Limbo in just five-ten minutes (the only thing that keeps you from clearing Limbo instantly is the likely Forced Level Grinding you'll have to do to beat the final boss). The vast post-game content keeps the game from ending on such a disappointing note though.
Game Breaker - Metal Slime class monsters. Immune to magic. Immune to most status effects (with the exception of "waste a turn" effects). Utterly disgusting amounts of defense and agility. Great magic casting and mana scores. The only downside is they have jack and squat for hit points, the higher end ones having a single or double digit cap. Oh, and they level up on a level approaching the glacial. Still, make one your healer and not only will your other monsters never die, the enemy AI will constantly cause them to attack your Metal Slime, who will shrug it off.
Goddamned Bats - Any monster with a permanent status effect such as poison, or one that drains mana.
Moment Of Awesome - Your first time catching a Metal Slime. Your first time creating your favorite boss from the original games. Winning your first online tournament match. Capturing your first Size 3 monster in Joker 2 (monsters so big they count as a party in and of themselves).
The Your Princess Is in Another Castle ending of Caravan Hearts. You see the ghosts that keep Loto's memory alive walking in place at the corners of the screen, when suddenly three eyes appear in the middle of the screen. There's absolutely no explanation for this until you go back to the old dungeons and research just what in the world is happening, and even then you'll need to spend a few hundred hours grinding your way to a solution.
The island sinking in Monsters 2. Every chapter, more and more of the land sinks. Nobody is willing to leave. Eventually you have to cross water just to get in a building...
Player Punch - Most strongly seen in Caravan Heart. Only really apt if you played Dragon Quest I and/or Dragon Quest II, especially if you did so as a kid. Nothing comes close to calling the player OLD than seeing the kingdoms you once played through now in ruins.
Another example are the ghosts who hope to keep Loto's legend alive, even worse so the Dragon ghost who formerly guarded Lady Lora, who remains faithful to his master after all the time that had passed. He was only doing his job...
Polygon Ceiling - Combined with They Changed It, Now It Sucks. The move to 3D came with a number of changes that alienated some of the player base, including the removal of randomly-generated dungeons, a very slow movement rate for the main character (necessitated by a combination of the new over-the-shoulder viewpoint, and the DS's small screen), a removal of random battles, and a complete overhauling of the skill and capture systems. In short, Joker 1 and 2 feel VERY different from the aggressively retro Monsters 1 and 2.
That One Boss: In DQM2, the King Squid, a bonus boss that appears after you clear the Pirate World, which is under the half way point of the game. Despite the early access to it, it has stats similar to those of the final two solo bosses in the main game, as well as an attack skill in Tidal Wave that hits your entire party and can deal up to 150 HP in damage to each monster (at the end of Pirate, your HP is likely to be under 100 or just above it, and without some good breeding and grinding, your monsters are unlikely to get enough HP to withstand two Tidal Waves without resistance by the time you gain access to the final world Limbo). It additionally has Pani Dance to spread the dangerous confusion status among your entire party, a monstrous attack stat to hit you really hard without Tidal Wave, and a titanic 2500 HP to take a hell of a beating before going down (while being able to 2HKO your entire party in one move). You have no hope of beating it when you first gain access to it, and it'll take a very long time before you could reasonably stand up to it. Even by the time you can take it on without being crushed instantly, it'll give you a hell of a battle without a lot of over levelling.
That One Level: In DQM2, among the randomly generated magic key worlds, the "Lord" keys are especially horrifying. These worlds are filled with only the ??? family monsters, known as Demon Lords (which are the final bosses of previous games, as well as "evolved" versions of them). They're all extremely powerful, while being pretty much immune to all status moves with loads of other resistances. Not only do you have Demon Lords running around everywhere, but you have incredibly buffed versions of these already incredibly powerful monsters serving as the world's bosses, and if your party was strong enough for magic key worlds to have 4 star end bosses, you will have one of the 4 star Demon Lords serving as the world's final boss (which are the strongest monsters in the game). While these worlds are incredibly difficult and dangerous, the Demon Lords provide an inane amount of experience, and as such, these worlds can be used for some very fast levelling.
They Changed It, Now It Sucks - Caravan Heart's monster breeding system involves you beating up monsters until they drop a soul, then using 2 souls + 1 monster to make a new monster. This is partially because the monsters are characters in their own right in Caravan Heart. It was different, to say the least.
Then they went all the way with Joker, vastly lowering the monster count, adding Dragon Quest VIII style skilltrees, and removing all the randomization all in an attempt to make the game work in 3D. Joker 2 undid some of these changes — the traits and skilltree system remain, but there is much more randomization (although the maps are still static, the monsters in them are incredibly varied) and the reintroduction of palette swaps and the addition of monster size plateaus pleased the fans.
Joker 2 Professional added 100 extra monsters to the 300 or so, making about 420. (On the other hand, some view this not as a bad thing in and of itself, they're just upset that they never got to see all the cool new stuff).
Even worse, Strongers were removed from the main list, since every monster could do it. This made about 1260 monsters - even Battle Road Victory only had 800!
Wasted Song - In DQM 2, the aforementioned Limbo has its own unique overworld music, but unless you spend a lot of time grinding with Limbo's random encounters before fighting the final boss, you'll get to barely hear it, as the world is extremely short as covered above, and once you beat the world, its overworld music reverts to the generic overworld theme that plays in the rest of the main worlds, which then makes the Limbo overworld theme Lost Forever on that file. The theme that plays inside the final boss' castle, a remix of the Limbo overworld theme, also qualifies, as unless you grind against the random encounters there, you'll also only hear it for like five minutes before fighting the final boss, and once you beat him, his castle disappears permanently, making it inaccessible for the rest of that file (you can still enter the entrance area before the castle after beating him and hear the castle's theme, but the bridge to the castle is gone and there's absolutely nothing to do there).
The Ice World also has its own unique overworld theme that plays nowhere else in the game, and like Limbo, it turns to the generic overworld theme after clearing the world, making it permanently gone on that file upon clearing the world. However, unlike Limbo, the Ice world is very large and elaborate with a lot to explore, so you'll spend some time there with its unique overworld theme before clearing it.