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.
Breather Boss: While each previous bosses all required a specific strategy to beat them, the Panda King is nothing short of a walk in the park. His attacks are all slow, blatantly telegraphed (not that that's a bad thing) and easy to avoid. And what do you have to do to beat him? Avoid his attacks while approaching him, then whale away on him with your cane. No other boss had you do this. At all.
The Computer Is a Cheating Bastard: In "Burning Rubber," Murray must drive the team van over a series of computers that fall from the ceiling. This wouldn't be so bad, if not for the Fire Slugs that are also trying to destroy them before you can reach them. You never know where the computers are going to fall, and there's no adjusting the camera to find them—but the Fire Slugs are able to immediately head toward them. Some of the Slugs even spawn directly on top of the computers, and even when they don't, they can still appear without your knowing it and destroy five or six consoles before you can respond.
Moral Event Horizon: Clockwerk definitely crossed it when he had the Fiendish Five kill Sly's father, but it turns out that his whole reason for it? He hates the Cooper line and wants to surpass their reputation.
Doesn't help that in the PS3 Collection the rhythm that you have to press the buttons in is off beat from the song. It can really annoy some of the more musically inclined players.
Muggshot as well, mostly because you have to have amazing dodging skills in order to dodge his bullets, and Muggshot is a pretty fast mover. To make matters worse, the second floor is oddly shaped with a few pitfalls and the last phase of the boss has you spire jumping to turn the mirrors around. Oh, and if you use a mirror as cover that's been turned to face him, it'll turn away when he goes to shoot you.
It helps when you figure out that the second stage is shaped like a dollar sign.
"A Ghastly Voyage", in large part due to the dozens of obstacles you have to shoot through to advance, not to mention all the Goddamned Bats.
"Sinking Peril". Remember that Daredevil Comet mission from Super Mario Galaxy where you have to rush through Melty Molten Galaxy without getting hit? Yeah, it's like that, but with a lot more climbing and precision jumping required.
Not to mention the fact that there are portions of the level (electrified gates) that you can't rush through—you have to wait for the electricity to stop firing before you can pass.