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.
Plus, that's around the point where you stop getting ability upgrades and instead just get near-useless Gold Orbs for completion of an area. Needless to say, the game begins to get rather boring when every single area ends with "Ooh, I wonder if I'll actually get an ability this time around...no, it's just another orb. Yipee."
Goddamned Bats: Any and all ghosts. Also the laser turrets in FireCage. Most of the enemies are pretty easy to deal with, though.
Goddamned Boss: The boss of Skysand. Its "Bullet Hell lite" isn't too bad, but you have to defeat it by making its floating sword hit its body, but the sword's attack phase occurs in long intervals, and the body is hard to hit. And every time you want to fight it, you have to traverse a few rooms which require some training to get through unscathed and in which everything deals heavy damage. It feels very satisfying to finally defeat it, though.
The boss of the Curtain, Shakespeare the ninja , fits this trope to a T. He's not too difficult to defeat, but dying to him means you have to go back through multiple screens of difficult platforming because of the Checkpoint Starvation in that area.
Nightmare Fuel: The purple ghosts- doglike purple beasts that are invincible, appear randomly, and their appearance is marked by the screen turning purple and the music changing. If a bird leaves Skytown, these things come for them. Two birds get carried off before your eyes by these monsters. After seeing them in cutscenes, they're able to come after you.
DarkGrotto is pretty terrifying if you're not expecting it. The bodies of four bird explorers who came before you—and got annihilated by the ghosts and DarkRed—can be found down there.
Paranoia Fuel: After a certain event occurs, there is a particular type of enemy that can appear out of nowhere at any time. It is large. It is invincible. It chases you regardless of where you are on the screen. It passes through walls. Sometimes, more than one of it appears at a time. Its appearance is accompanied by the screen taking on a shadowy purple tint and the music becoming creepy. About the only (admittedly, large) comfort is that you can get rid of it by simply traveling between screens.
Actually, two events can cause that enemy to start appearing. If the ghosts make you paranoid, you might want to avoid SkySand and FireCage for as long as you can.
If you're playing on Simple, the ghosts will never appear except in the final area in set locations. so there's nothing to worry about there.
Surprise Difficulty: Don't let the game's charming feel and MS Paint visuals fool you: this game was created by the same guy who created the Jumper series, and the difficulty from those games certainly carry over into this one.
That One Boss: Some of the bosses are pitifully easy. Some of them are... not.
Dark Red and Fluffy in particular. In order to beat Dark Red, you need to deflect two (or more, on higher difficulties) of his fireballs at the same time. Which is only possible if you're two platforms away from him, and it takes a bit of practice to do that. And even then, you can only let the shot loose at the last second. Not to mention he's in a Blackout Basement... Also, Fluffy. Wouldn't be so hard except for the fact that you can only stand on a few small platforms that constantly fall when you're on them. And falling off makes you suffer huge damage. Injuring him is no easy task either.
The second battle with Shakespeare the ninja is no fun either. Little twerp keeps teleporting in right on top of you to wing his gear shuriken at your head, and the ground below your little platform is spikesville.
StoneEye. Have fun dodging a ton of projectiles and energy barriers in a shaft that's only a few times the width of your sprite.
FireMachine. Try jumping through gaps in an energy beam when there's four of them at once, some that open and close rapidly. And a bunch of projectiles, too.
That One Level: The final dungeons, full stop. Aggravating, especially difficult spike mazes that dish out huge damage and are tough to see, let alone avoid, and a very small number of save points means that even the slightest mistakes will cost you dearly. And on harder difficulties, there are even fewer of them.
The Curtain also qualifies, especially if your platforming skills aren't up to snuff. The slightest mistake will set you back several screens. The challenge of the area combined with it's Checkpoint Starvation will make you want to tear your hair out. It doesn't help that the area is pretty much just as hard on Simple as it is on Difficult.
That One Sidequest: Getting the best reward from the RainbowDive minigame. Not only do you have to collect practically every star, you have to utilize a quirk the game never tells you: shooting a star earns 15 points. Fail to collect them and you have no choice but to run all the way back to the save point at the bottom and warp back up to try again.