1. Excessive Blocking

Why It Sucks When Other People Do It: You have to beat everyone in this room before you're allowed to leave. The first 49 minions of the Dark Bad Power Guy were slain without too much effort, but then this asshole shows up. And blocks. And blocks. Sometime he doesn't NOT block unless you hit him in the back. Too bad he never turns around.
A gimmick like this can turn what should be a simple enemy into a nigh-insurmountable challenge. Not that there's anything wrong with that, but when you have 6 nigh-insurmountable challenges surrounding you, all of a sudden Easy Mode starts looking pretty prime.
Why YOU Have To Do It: Part of the fun of platformers is your remarkable prowess. You can take out a room full of enemies if you do it right.
Problem is, just because you can kick copious amounts of ass doesn't mean you can put up with a lot of damage. Hitting a common opponent a dozen times isn't that odd at all, but getting hit a dozen times is always bad news. If you don't block, you die. Plain and simple.

2. Frequent Healing

Why It Sucks When Other People Do It: When you have an enemy on the ropes, it's a good feeling. Clobber him one or two more times, and sweet victory is yours. Plus, once you beat this level, you get to use the Homing Groin Fireball. That's gonna be one sick-ass spell, and it's yours after one more sword slash.
Suddenly, those all-too-familiar green sparkles surround your enemy. Maybe he did it himself, maybe some big fat semi-boss did it for him. Either way, he's back to 100%, and your Homing Groin Fireball goes down on your to-do list. Right below 'the same damn thing you just did a second ago'.
Why YOU Have To Do It: More often than one might think, you don't even have to be in a room with enemies to get hurt. Any room with puzzles or traps, or even long leaps over steep drops, can chew through that precious health bar even if you only screw up once or twice. But eventually you get past the puzzle room – with only a sliver of health, but you made it, and you are NEVER going back in there again.
Suddenly the beasts surround you, and one manages to scratch you with his fingernail. And your last checkpoint was, naturally, right before the puzzle room. Healing potions, turkey legs, first aid kits, what have you, you're gonna need 'em if you wanna live long enough to actually do any fighting.

3. Combo Breaking

Why It Sucks When Other People Do It: This is a tough combo. But if you time your three light attacks, two heavy attacks, two more lights, a jump and one last heavy just perfectly, you perform the "Break His Kneecaps And Bury Your Sword In His Face And Also He Explodes" combo. Damn near impossible. Totally epic. Totally worth it.
But that son of a bitch has one of those built-in defense systems governments would literally pay for in human lives: get hit five times, and the sixth attack gets automatically blocked. Now whose kneecaps are broken? Wherefore art thou, face for sword-burying? How come this guy is all non-explodey!?
Why YOU Have To Do It: Most platformer enemies, thankfully, don't have a very deep move set. A slash, a kick, maybe a throw. But when you get one that has a combo, it is always a brutal one. Usually a fatal one, unless you're playing on easy mode or just healed up.
It's not easy, but you had better damn well find a way to break that combo, be it blocking, dodging, or using a spell or ability that can interrupt. That last option is especially wonderful because that's what he gets for hitting you FOUR times! The nerve! And he was gonna try and do it again? Have you no shame, skeleton that just got fried by a lightning bolt?

4. One-Hit Wonders

Why It Sucks When Other People Do It: Okay, this guy looks pretty tough. But it's fine. You haven't quite mastered the dodge, but you got a couple health upgrades, and you just gained a sweet new weapon. Even if he hits you once or twice, it's not that big of a deal. You've got some potions, and you can dish it out as well as you can take it.
Wait, what the hell was that? He hit you one time, and you totally just fell over. No, you're not getting back up – he straight ended you in a single blow. No, you didn't forget to heal, this guy is just that brutal. Yes, he'll do it again. Better practice that dodge.
Why YOU Have To Do It: Usually, there aren't too many enemies that you can kill in a single hit – they tend to get stronger as you do. But if you find a few, it is time to impose your dominance. No, you probably won't intimidate the guy who just smote you in one mighty swing of his blade, but you certainly will feel better knowing that these chumps were effortlessly laid low by your incredible might.
Fortunately, enemies capable of the dreaded one-hit wonder are few and far between. They're almost always bosses, and most of the time there's a warning sign of some sort – rearing back or glowing, something like that. But if you're stuck in mid-combo, and can't move until your stupid ass stops swinging at nothing, well, he's gonna make you regret trying the ol' BHK&BYSIHF&AHE on a foe of his caliber. You really should have known better.

5. Continues

Why It Sucks When Other People Do It: Ho. Ly. Crap. That boss fight took 3 and a half days. You used up all your healing potions, you're out of mana, and half your life bar is gone. But you did it! The Ultimate Evil is slain, and you have saved the world. Now just exit the room – ooh, there's gonna be a cutscene. Settle in and watch, you've earned it.
…You have got to be kidding. He's not dead. What's worse, it looks like he was just screwing with you before. Because now he's getting up, he's twice his previous size, and he is far more spiky and shadowy and scary than before. And you have half a life bar, no items, and only the faintest glimmer of hope. Oh, never mind, you're dead. Goodbye, glimmer. We barely knew ye.
Why YOU Have To Do It: Of course you have to use continues. Have you ever played all the way through a platformer on any mode besides Easy and never died once?
Didn't think so.