Annoying Video Game Helper: Gadd constantly calling you to critique your work is probably the only universally agreed weak point of this game. It gets annoying at times and he does it so frequently that it really slows down the pacing of the game, making him an almost unbearable chatterbox on repeat playthroughs.
The Brain, the final boss of the Scarescraper. Like all the other bosses, he's a King Mook with a special gimmick; in this case, Poltergeists. The Brain's gimmick is being able to charge up and release a sphere of energy to free himself from vacuums. However, Poltergeists already have the ability to attack while being sucked up, the charging can be interrupted by a power surge, and the energy sphere does no damage, giving no incentive to simply break the charge. In the end, he ends up being the second weakest of the four Poltergeist types.
For E. Gadd, since in the original game he states he was turning ghosts into portraits for his art collection, which makes him come off as a Designated Hero since not only is there not any moral reason for it, but many of the ghosts he went after weren't even violent unless provoked. In this game, most of the ghosts are friendly until the Dark Moon is shattered, and after it's reassembled and the ghosts go back to normal, he releases all of them.
One of the first game's most major flaws was its short length. Thankfully Dark Moon was quick to fix this.
Best Boss Ever: The very first boss: Grouchy Possessor (aka the one who possessed the spider queen), as well as the Final Boss King Boo. While Grouchy Possessor has the same weakness (lighting the web ball that hangs from the spider's web) that forces it out of its host, it changes the way you attack that weakness every time you damage it, while King Boo alternates between fighting you for real, and forcing you to run through an obstacle-filled course with him right behind you.
Despite being more critically well-received than its predecessor, fans of the series are split on two of the game's major elements: the mission-based gameplay and the game being taken place in 5 different mansions rather than a single location. Many fans who came into the series this installment think the mission-based structure is fun, or at least excusable given that the game is on a handheld and made with portable play in mind, and that the various mansions offer a great deal of variety. Meanwhile, most fans who started with the first game feel that the mission structure stifles exploration to an unacceptable degree.
As pointed out by Chuggaaconroy, the boss fight against the Shrewd Possessor in the Secret Mine is this. Some people enjoy the boss fight because of its uniqueness and the scale of the fight. Others find it to be frustrating and overly difficult.
Critical Dissonance: Critics lauded Dark Moon much more than the first game, citing its inventiveness, graphics, length, and gameplay to be superior to its predecessor. Fans however believe that while the game is great in its own right, consider this to be the Oddball in the Series and find it weaker than the first.
Complete Monster (includes Luigi's Mansion 3): King Boo sheds any of his redeeming qualities from the first game and becomes an unfettered monster. After his first defeat by Luigi, King Boo is consumed by his lust for revenge, devolving into an uncaring sociopath who would willingly kill everyone and destroy everything just for a chance to get back at his rival. In Dark Moon, King Boo shatters the Dark Moon, enslaving the minds of the once-docile ghosts of Evershade Valley to his control. Unleashing them on the valley, King Boo abducts Mario himself and several Toads and imprisons them within paintings. Uncaring about his minions, and in a stark contrast to his characterization in the first game, King Boo performs experiments on his ghosts against their will to amplify their destructive potential, and calmly allows his own Boos to be caught and sealed by Luigi. After Luigi gets close to restoring the Dark Moon, King Boo starts to lose it, and tears open a portal to the ghosts' homeworld with the intent to destroy the universe as an act of spite. Once he's foiled, King Boo warps Luigi to his own dimension, announcing his intent to trap Luigi and all his friends in paintings as he conquers the world and "paints the town red". In 3, he is set free by Hellen Gravely, and together they gather Luigi and his friends in the Last Resort hotel in another attempt to trap them in paintings. When Hellen and her staff are all captured, King Boo dismisses them as worthless failures and eventually tries to absorb the entire hotel, including all the captured ghosts and his own Boos, into a group painting just to capture Luigi.
Contested Sequel: The game gets this, with some preferring it to the original thanks to its improvements in length and variety, and some others preferring the first game even when stating this one is a mechanically better game due to its atmosphere.
The unpossessed spider queen is actually kinda cute. Still a little unsettling due to being a spider, but cute.
The Boos (besides King Boo, of course) are rather adorable as well, with their pun-tastic names and signature high-pitched cackles. There's that little *squeak* they make when you vaccum them up as well.
Demonic Spiders: While Gobbers are usually minor nuisances, they become this in Scarescraper's Rush Mode. There, they appear in certain rooms where whenever you try to use a predetermined door, one will show up and cause goop to appear on the door, with the only way to unlock the door being defeating it. Goobers have at least 100 HP at all times, and in Rush Mode, the time limit is so strict that you might run out of time. Even in groups, they're more likely split up in order to look for the goal, and by the time they regroup, time's up.
Even Better Sequel: The first game is fairly beloved, if often criticized for its short length. By contrast, the sequel has enjoyed almost universal praise.
"Funny Aneurysm" Moment: In one of the missions for the third Mansion Dr. Gadd tells Luigi to stop trembling in fear whenever he uses the teleporter, saying that they've "done this a dozen times" and there's nothing to worry about. Of course, he's proven wrong at the end when King Boo is able to sabotage it.
The little orbs that float around creating illusions. They have a tendency to scatter when they're revealed, will move around if they're not vacuumed up quick enough, and can hurt you if they pass through you. And if even one goes back into hiding, you have to do the whole process of revealing the hidden object and vacuuming them all up again.
The actual bats, along with spiders, mice, and beetles are perfect at harming and distracting you while you're busy sucking up the more dangerous ghosts.
Goddamned Boss: The Terrible Teleporters in the Scarescraper. A Dual Boss based off the Sneakers, their gimmick is spawning little funnels. These funnels will teleport you to another room, forcing you to walk all the way back each time. You'll only lose a few seconds generally, but it's still annoying.
Moral Event Horizon: King Boo crosses the line when he destroys the Dark Moon, knowing that he'll force every ghost in Evershade Valley under his control, then creating a portal to unleash an army of ghosts to tear apart the fabric of the universe, and finally by putting Luigi through a horrific experience before they fight and gloating that Luigi, Gadd, and all of their friends are going to suffer the fate Mario did, which he's shown to be perfectly capable of doing.
Scrappy Mechanic: The occasional balancing sequences which require you to use the finicky gyroscope to keep Luigi from falling off the beam or rope he's walking on. Made worse in the first mission of the Secret Mine, where gusts of wind will force you to adjust your balance quickly.
Sophomore Slump: While fans still argue that it's a great game in its own right, this game is generally agreed to be the weakest game in the Luigi's Mansion trilogy due to its mission-based structure, lack of portrait ghosts and its divisive "wacky" art style.
The Spider boss/Grouchy Possessor from the first mansion due to being a brutal Wake-Up Call Boss. It is a Puzzle Boss that requires you to move a wad of sticky web through an obstacle course to a fire source and if you get hit, you have to start over. It is also quite easy to miss the fire sources you are given for the second and third hits. Doubles at Best Boss Ever for some.
E-4's Big Boo boss if you're trying to get a 3-star rank on it, as you have to finish it within 3:20 without taking damage (or within 1:20 if you get hit exactly once). This can be an exercise in frustration as it means you have to trap a lot of Boos every time you manage to split up the Big Boo and avoid the other Boos' attacks while you're trying to aim a Boo at a cage. About the best thing you can do to ensure that you'll have an easier time is to try to catch as many Boos at once with your Darklight before you aim for the train.
Among the Scarescraper bosses, the Primordial Goo type bosses. Based on Creepers, they behave almost exactly the same, with the major difference that whenever a power surge is used, the Goo will fire a regular sized Creeper right at the Luigi who used it, and if he doesn't move out of the way, the Creeper will trap the Luigi, dealing a good amount of damage, and free the Goo, wasting time.
The Ice Monster controlled by the Shrewd Possessor in the D mansion. You have to master using the 3DS' Gyroscope and overall terrible circle pad aiming to destroy pieces of the boss' armor using finicky cannonballs. If you screw up 6-7 times in a row his armor will re-freeze so you have to do it all again. To top it off it is possible to overheat your cannon resulting in a instakill. And guess what? You have to do it all over again. That's not even getting into attempting to 3 star it.
The OversetPossessor. His possession choice? The arena itself, giving you no specific target most of the time. His main offense? Zerg Rush waves of prior enemies (including some Demonic Spiders) that even repeat if undefeated within their time limits. When he finally comes out to fight directly? His Paranormal Shield No Sells the Poltergust, making you wait for an opening first. And those far-reaching clock hands? They can hurt you, too; brace yourself for being dragged into them over and over.
Another nasty group of bosses on the Scarescraper is the Beetle Whisperer/Scarab Nabber group. The King Mook to Slammers, the main gimmick of their battle is the ability to generate up to five beetles that fly around the room. These do not do that much damage, but they will stop you from vacuuming them if hit. This does not sound too bad, except that these types of ghosts have regenerating Paranormal Shields, forcing you to break the shield again every single time the stream is broken. They tend to summon Sneakers to help, which also break any capturing attempt if you don't dodge their scares at the right time. Five minutes might seem like a generous amount of time, but the shields, constant beetle swarm, and assistants will likely do a pretty good job at draining all of it.
E-5. The level is simply running from room to room, catching the ghosts. However, you're on a time limit, and there are tons of ghosts. And once you beat them all, one final wave appears. This one is massive and contains dozens of ghosts who can easily gang up on you. Hope you practiced capturing multiple ghosts at once, because taking them on one at a time is an easy way to get yourself beaten up. Oh, and this is the one level in this mansion where you have to go out of your way for the Boo, meaning if you don't get it, you have to do the level all over again. There's a video here that shows one way to do it. (He does show you how to get the Boo first, which is definitely a good idea.)
E-4 if you're trying to get a 3-star rank on it, which in this case requires you to beat it within 3:20 without taking damage (or alternatively, within 1:20 if you get hit exactly once).
Rush Mode in general if you don't know how to find time pieces, or deal with the door-blocking Gobbers. The worst part is that everyone has to reach the end room, so it only takes one person who doesn't know what they are doing (or one griefer) to end the game for the entire team.
B-6. Those stairs are the absolute worst and you know it.
They Wasted a Perfectly Good Plot: After beating the Spider Queen/Grouchy Possessor, E. Gadd makes a big deal out of the fact that possessor-types can possess and control living beings... which is never used again, considering the following bosses are a staircase, a clock, a chunk of ice, and suits of armor.
Unwinnable by Insanity: You can lock yourself out of Treacherous Mansion if you play around with ghosts, teleporters and objects in the Nautical Exhibit. But you have to be really, really trying to pull this one off.
What an Idiot!: E. Gadd. While he's clearly not there mentally throughout the game, the fact that he sold the King Boo painting at a garage sale really shows it. True, the Professor was led to assume that King Boo would need to be sent back through the Portraitficationizer, like Mario was, to be restored (little did E. Gadd know that Spirit Balls were kind of a thing King Boo could use), but being so careless with something so dangerous is still pretty bone-headed of him.