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.
YMMV: Zelda II: The Adventure of Link
Breather Boss: Barba has such predictable and easily avoidable attacks that you'll be more worried about falling than getting hit.
Assuming to know how to kill him, Carock is another, and if you have enough magic for the Shield spell, he goes from pretty easy to a complete joke.
Ironically, Dark Link, the final boss. The Thunderbird's attacks cause insane amounts of damage and are very difficult to avoid, plus its movement pattern and small target area make it a challenge to hit in return. Not at all helped by the fact just to damage it requires blowing half your magic bar on the Thunder spell. By contrast, Dark Link is more or less an Iron Knuckle who can jump (though not nearly as much or as high as the Bird Knights) and has more health. And he doesn't beam spam. He becomes even easier if you know the trick of positioning yourself in the far left corner on the arena.
The Red Dairas are the prime example of this trope. They are found in Death Mountain on your way to get the hammer. You'll likely have only one heart upgrade and their throwing axes deal a lot of damage, even with the Shield spell up. Oh, you can't deflect their projectiles either because you get that spell after you get the raft.
The appropriatelynamed Fokkas (bird knights) in the Great Palace. They do huge amounts of damage to Link (1 1/2 bars of his life meter) even at full life power, have the movable shield and shooting sword of the Iron Knuckles, and they love to take flying leaps that will invariably send them smashing right into Link. They're also quite durable. And if you beat them, for some reason the experience points gained is downright paltry (70 for a red one, 100 for a blue, contrast 150 for a blue Iron Knuckle, 200 for a blue Lizalfos)
Ironknuckles, particularly the blue ones. The orange and red ones aren't so bad once you get used to them and get better attack power, but the blue ones never cease to be a pain. Not only do they fire Sword Beamsat an alarming rate, but they back away from you as they're doing it, making it difficult to land hits! The mounted ones you face as the boss of the third palace and as mini-bosses in the sixth are especially infuriating as they'll retreat Behind the Black, leaving you to wait for them to show themselves!
Lizalfos in general, but especially, again, the blue ones. Orange lizalfols are like Ironknuckles, only, the spear they use is faster than the Ironkunckle's sword. The red ones use unblockable maces. Blue ones throw unblockable maces, combining the worst traits of ironknuckles AND dairas. At least by the time you fight them you'll have learned the Reflect spell.
Dark Link is also surprisingly popular. Some even consider him a character in his own right and give him a personality in fanworks.
Goddamned Bats: The two different animated head statues, Ra and Mau. (Ras are the dragon-shaped ones, Maus are the panthers). Both spawn infinitely and drain experience when touching Link. Ras double as Ledge Bats, Maus basically Zerg Rush Link.
Any enemy that drains your XP when it hits you. They tend to respawn, as well.
Good Bad Bugs: Hold left and right on the D-pad (normally impossible unless you have a broken D-pad or are using a keyboard), and release. This will usually cause Link to slide off at high speed in one direction. Demonstrated to beautiful effect in this TAS (though good luck doing it this well in real-time).
Note, however, that using up and down at the same time (which one might assume would have similar effects) is not always a Good Bad Bug; used in an elevator shaft, it causes the elevator to ascend rapidly, but it can easily crash the game or put Link in an Unwinnable situation if a number of things happen. Players are advised to keep save states handy when testing it out.
Mis-blamed: Zelda II's hatedom didn't form until well after the game had been released and all of the (non-3D) sequels retained the top down perspective. Back in the old days, Nintendo of America felt that Mission Pack Sequels wouldn't sell in America, and so sequels were often very different from the original. See also: Super Mario Bros. 2.
It's also the first appearance of many names from said game, though continuity wise the towns are named after them.
This game is also the first to feature a grown up version of Link. He is canonically sixteen.
It's also the first game where the Triforce of Courage appears, the triforce mark on the hand is mentioned (but unseen in game) and even the first game with the three pieces of Triforce reunited.
This was one of the first games to have a New Game+ as it's currently thought of.
Paranoia Fuel: The man in Saria who proclaims, "EYES OF GANON ARE EVERYWHERE, BE CAREFUL." Its supposed to be a hint that some villagers will turn into enemies if spoken to, but new players likely won't understand until they uncover a spy on their own.
And Rebonack. Blue Ironknuckles are a pain to begin with, but Rebonack has the habit of backing out of the screen where Link can't stab him.
That One Level: The Great Palace qualifies even though it's the last level (and therefore held to a higher standard). If you actually even made it to this dungeon, then this palace will undoubtedly make you rage because of how complex and confusing it was. There's a particularly devious trick needed to find the bosses of the Great Palace. A section of the place has floor covered in a single layer of breakable blocks. There's a hidden warp pit one block long under one of them. Link can walk right across a pit that size without falling down it, which means the player might walk right over it and never notice.
Though mercifully, if you get a game over in the Great Palace, you respawn at the entrance to the palace.
Death Mountain, which apart from the Great Palace is the area that gives players the most trouble, and you have to pass through it very early the game. Amongst other things, it involves a maze on the overworld that's easy to get lost in, along with hordes of enemies.
Scrappy Mechanic: When Link dies, he is sent back to the palace where Princess Zelda sleeps, meaning that Link must traverse the landscape again to go back to the dungeon (except the final dungeon). If that dungeon was far away from the starting point, then have fun trying to get back there. To make matters worse, losing all your lives not only resets Link's experience back to zero, but doesn't bring back any extra lives he obtained!
Woolseyism: As usual, there are some differences between the JP and US releases.
New color palettes and tile sets for the different palaces (they all use the same gray stone pattern from Parapa Palace in the JP version).
The wandering monsters were originally little balloon things.
New more varied music.
The bosses Carock and Volvagia are now animated instead of merely static sprites.
The addition of the classic Item Get pose (before, stabbing things to get them like with the potion was done with all items.)
There's a brand-new boss in place of a second fight with Helmethead.
For better or worse, the legendary difficulty is worse in the US. Tektites only being vulnerable to the Fire Spell (and first encountered long before you get it!) is new to the US, as is XP-draining monsters.
The amount of XP needed to level up in Japan stops increasing at 4000, not 9000, which means you're a lot tougher by the time you reach the Valley of Death and the Great Palace, which are ultra-deadly in the US.