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.
Ensemble Darkhorse: Ask an old fan who their favorite Portrait Ghost was and there's a good chance it'll have been Area-spanning ally Madame Clairvoya or pretty young Gamer Chick pianist Melody Pianissima.
Fridge Horror: All of the Portrait Ghosts are all family members and servants. However, all of them died, including some children. Plus, the mansion was claimed to have been built for the purpose of fooling Mario and Luigi, yet the Portrait Ghosts seems to be familiar with the place.... is King Boo as much of a monster as Dimentio?
Goddamned Bats: The ghosts that appear in the hallways. They're annoying, often pop out of nowhere, yet technically have zero health points (normally you have to battle ghosts to defeat them, but a ghost with 0hp will die the moment you attack).
Jerkass Woobie: Chauncey, the first boss who is fought after you defeat his mother and father. That's right, a baby is trying to avenge his fallen parents, which kind of makes you rethink the entirety of the first boss fight and makes it much Harsher in Hindsight as well.
Paranoia Fuel: Some doors in the mansion are fake and will slam you into the wall while a ghost laughs at your pain. Some of the doors are more obvious, like a trail of coins leading to them. Others give no sign that they are a trap, and can startle the hell out of you if not expecting it. Luckily, you can spot them by either vacuuming the doornote If it shakes, then it's real. or using the Game Boy Horror note The fake doors don't appear on the map., then destroy them with fire. Also, false doors lack doormats.
Boolossus. The Boos that compose it start off fairly easy to catch. You have to freeze all fifteen Boos and capture them to win. Once you hit a certain number (usually around 7 or 6), they begin to avoid your ice blasts and will make more of an effort to harm you. Those alone are difficult to catch, but when you're one on one with the very last Boo, it will continue dashing at you and dodging your ice until you hit it. And you have to survive the fight without taking too much damage, whereas in most Portrait Ghost fights, it's detemined on how long you hold them in your vacuum suction (in boss fights, the picture frame is determined by how much health you lost during the fight). This one Boo will outlast your health and quickly deny you that gold frame (if you have not lost enough health already earlier in the fight), unless you somehow manage to hit it during its dash attack.
The final boss, King Boo. For starters, his Bowser Suit has quite a few more attacks than most of the other bosses in the game. He can toss explosive spiked balls at you, suck you into his mouth, breathe fire, and trample you. When you blow the suit's head off, his head can fire paralyzing ice balls as you are trying to pull King Boo in for the kill, which can screw you over if you are going for the gold frame here. When King Boo's health is lower than about 200, he will put his Bowser head on backwards, which results in him running all over the place sporadically. King Boo himself has only one attack if you let him fly about, which is trying to ram you, as is typical of Boos by this point. Plus, there are pillars that are littered around the arena which can be either a help or hinderance to your folly. But when you beat him once, you can beat him any time after that provided you keep on your toes and avoid the attacks effectively.
Vincent Van Gore. He can make trios of ghosts, which can either corner you or knock you out of a capture attempt. To make this even worse, some of these ghosts are either Goddamned Bats or Demonic Spiders (the grabbing ghosts and the blue ground pounding ones come to mind), which, if you're not careful, can easily drain your health. Thankfully, Vincent Van Gore himself is utterly pathetic (he doesn't even attack you), but it boils down to the fact that this is a fight you'll either quickly win or quickly lose.
Bogmire and Chauncey aren't quite walks in the park either, especially if you're trying to get the highest rating on them. Chauncey can summon a lot of attacks after you. The balls he summons are the worst, since their paths can be a bit unpredictable. The horses aren't better, but you can goad them by running towards them and past them as they fly by, missing you completely. Also, when Chauncey flops about on the bed, steer clear or else kiss a huge chunk of your health goodbye. Bogmire is probably irritating for all of those shadow clones it sends after you.
Sir Weston, when you're going for a Rank A. His icy room makes it impossible to get a good traction on the floor, so getting a large pearl to drop is mostly by luck. Oh, and you have the icicles falling from the ceiling constantly, so having one drop on you just as you're about to finish him with a perfect run can ruin your day.
The Clockwork Soldiers. They aim their popguns at you, and the blue one drops the pearls.