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.
Crowning Music of Awesome: Chef Boyardee provides his awesomeness with Non-Human. The first stage of STREEMERZ also qualifies.
The soundtracks from Sombreros, Illuminator,Bubblegum Rossy and Meong certainly qualify aswell.
Demonic Spiders: The disembodied mouths in Non-Human. Their tongue lash hurts a lot and has longer range than the mechs' regular attacks. Normally you can jump it, but you'll usually have to retreat as they tend to appear enclosed spaces. They also have quite a bit of health, which means that you'll need to advance/retreat a few times to kill them.
Game Breaker: The Bug-Torcher 5000 in City of Doom. Deals huge damage (1-2 bursts will kill a window bug) over a large, medium-range area as well as destroying enemy projectiles. Handy for the shot-spamming first boss. It guzzles ammo, but this can be solved by tapping the attack in bursts.
Goddamned Bats: The flying disembodied hands in Non-Human. These things fly upwards out of regular weapon reach, then do a diagonal rush towards the player. They hurt a lot and can take quite a bit of damage for a small hand, and worst of all, they attack in groups!
City of Doom has the "window bugs". They take a good bit of hits to die, hide in windows, and only become vulnerable when they pop out and fire three aimed shots. Thankfully the Bug-Torcher 5000 incinerates them with great ease.
Moral Event Horizon: Dr. Murdon in Non-Human does this during the introductory cutscene. The heroes are closing in on him, so he pumps mutagen into a nearby village and turns all the innocent villagers into the titular monsters.
Paranoia Fuel: Illuminator's gameplay is based in this. Killing enemies requires keeping your flashlight off until it charges up to a certain amount, so knowing where the enemies are is difficult. Even with all the lights you can place to help keep track of enemies, there'll still be plenty of times where you turn your flashlight on and fry an enemy who was only inches away from you and you didn't even know it was there.
Squick: Gustav from Non-Human. He coughs up his guts as an attack.
The Rolling Rollblax in Silver Sword, which forces you to constantly be on the move, and drops mushrooms whenever it's hit, exactly where it's hit, which can very quickly clutter the area and make it hard to move if you don't carefully plan where you hit it. And just leaving it alone while you clean up the mushrooms isn't an option, either, because it quickly picks up speed unless you hit it to slow it down.
The final boss the irradiated baby princess counts as well, being a Bullet Hell boss with a tiny, tiny hitbox, which is an absolute nightmare when you can only have one projectile on the screen at a time, let alone while planning the distance your Sword Beam travels to exploit it's ever-changing weakness. The third phase is nigh impossible, due to having to maximize your distance on a tiny, moving target that never, ever wants to just move in the path of your Sword Beam.
The Giant Enemy Crab/Bug thing in Non-Human. It has two heads as the weak points, the top fires out a bouncy energy ball while the bottom shoots one that hugs the floor, then moves up when you jump over it. Both of these attacks hurt a lot, are tricky to avoid, and the boss can use both at the same time. Then there's the fact that the boss is constantly advancing towards you and limiting your space. Your solution is to shoot the heads to damage him and push him back, easier said than done because the crab can use his claws to guard one of them!