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: Cut Man and Bomb Man. Cut Man's attacks are sluggish and easily dodged, and Bomb Man is more likely to kill you through Collision Damage than he is with his bombs.
Breather Level: The third Wily stage is very straight forward with no real gimmicks.
Demonic Spiders: Big Eyes will be the bane of many players' existence. Three hits from one kill you, they have two jump patterns (and only one can let players get past it) and they take 20 buster shots to kill. So of course they put one at the end of the stages of 2/3 of the game's Robot Master roster AND put three in a row at the beginning of Wily Stage 1.
Ensemble Dark Horse: Guts Man is often considered the most iconic Robot Master from this game, if not the entire series. Not surprisingly, many people were pissed that he didn't make it into Dr. Wily's Revenge for the Game Boy (and, by extension, any of the Game Boy games).
Good Bad Bugs: The famous "Pause Trick", which allows you to rapidly pause the game while an attack like the Elec Beam is passing through an enemy, allowing them to take multiple bits of damage from the same shot. This is often used by gamers to beat Yellow Devil. In fact according to some, it's the onlyway to beat him.
Ladders are the most borked in this game (and Capcom could never quite get it right), making it possible to clip through walls and move faster than the graphics can load.
Elec Man, whose weapon is powerful enough to kill you in three hits from full health! God help you if you fight him without the Rolling Cutter.
Fire Man, if you don't know the trick to beating him. He is extremely aggressive, and his shots drop trailing fire right over your position as you jump over them, and the time they stay vary. If you don't know said trick, your only other hope is coming in with enough health and using Ice Slasher.
Ice Man can also kill you in three hits, and will likely do so if you didn't bring Thunder Beam with you and you aren't used to the timing of jumping through his attack pattern.
The Yellow Devil, a boss so tough that the Pause Trick, a trick where you pause when he's taking damage, is considered a totally genuine, guilt-free strategy for beating him.
That One Level: It's probably easier to list the levels that aren't that one level.
Elec Man's stage is a long vertical climbing stage, with the first part being the worst of it. The first room is a set of tricky jumps, followed by climbing up a long ladder while avoiding enemies that shoot at you. If you get hit at any point, you will fall down over half a screen before the game lets you catch the ladder. And once you get to the top, you have to hop over on disappearing blocks which if you do miss one, you can fall all the way back to the base of the ladder and do it all over again. You'll also have to do this again if you don't pick up the Magnet Beam.
Ice Man's stage, if you're doing it without the Magnet Beam. The first set of annoyances are jumping on disappearing blocks over Spines (those enemies that need a special weapon to defeat). Then you have to cross over a large pit with Foot Holders. Not only do they have an unpredictable pattern, but they shoot sideways randomly. If you get hit, not only do you have a high chance of being knocked off, but you can fall right through them. Even if you have the Magnet Beam, the game has a quirk where the top of the screen overflows to the bottom very shortly. If you climb too high, the game think you've fallen into a pit.
The beginning of Wily Stage 1, where you have to fight through three Big Eyes. It gets worse after that, not only is there a room with the Foot Holders with spikes on both the ceiling and floor, they have a tendency to go up to the ceiling when you're on them. There's also a room where the Magnet Beam is required. Thankfully, there's infinitely respawning weapon refill capsules just before that room in case you run out.
The final major platforming section of Wily Stage 2 requires dodging the cluster bombs from Bomb Man's stage. They don't leave a lot of room to not get hurt either.