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 Dungeon: Level 7 mostly features Goriyas, enemies from the first two levels who aren't much harder to kill than Moblins. In addition, the boss is Aquamentus, the same boss from Level 1, with absolutely no changes.
Level 8 is this in the Second Quest. While a bit convoluted in layout, the enemy pattern is the same as in Level 7 of the First Quest and the boss is just three Dodongo enemies with four ball-spitting statues in the room (note that another room in the dungeon has exactly the same layout). While a bit harder than a single Aquamentus, it's certainly less of a challenge than the Gleeok of the previous level.
Blue Wizzrobes. They do major damage, they're durable, they have an incredibly hard to predict motion pattern and you can't stand in front of them because they continuously fire spells if you do. And you can't stand in doorways to snipe them because they can become immaterial, pass through the walls and over the spot in the door, hurting you while they remain unharmed.
Orange Wizzrobes normally are easy enough to deal with, but they become quite dangerous in their own right when combined with Blue Wizzrobes. Trying to keep your distance from the blue ones tend to result in wandering into an orange one right as they teleport into your path, as one of the only advantages the weaker variety has is the ability to teleport a greater distance (the other being their spells which are twice as strong as those from the blue ones).
Blue Darknuts. They can't be attacked from the front, they're twice as fast and durable as Red Darknuts, and they do major damage.
Like-Likes. They eat your Magic Shield. They also frequently show up with Wizzrobes, where losing your upgraded shield makes you much more vulnerable.
Before you get the bow and arrow, Pols Voice. Unpredictable movement pattern, exceptionally durable, and they do major damage.
Red Bubbles, especially in the second quest. Rather than a temporary inability to use your sword, you have to touch a Blue Bubble or drink a potion. In either quest they also tend to get you killed by enemies that are primarily vulnerable to your sword.
Good Bad Bugs: In both the NES original and the GBA edition, immediately exiting and reentering the Level 1 dungeon will somehow unlock the first door, netting you a spare key. Hard to tell if this is an unintentional bug or a deliberate secret, though it was "fixed" in the Gamecube re-release.
Try to get through the second quest without looking at a map. Just try. You'll probably get to about Level 3 before giving in.
Starting with Level 2, the labyrinths in the second quest feature walls you must walk through (by holding down that direction for a couple of seconds) instead of bombing in order to proceed. There is nothing whatsoever in the manual or the game itself indicating this new twist in gameplay.
Most of the game, really, especially with the "Blind Idiot" Translation. Miyamoto outright admits that the game was designed to support player collaboration (ex. walkthroughs from more experienced players).
That One Dungeon: Level eight in the first quest and level seven in the second quest (inverting the entries for Breather Dungeon above) are full of Blue Darknuts, who are some of the hardest enemies in the game. Level 7 in the second quest actually requires you to kill no less than three rooms full of them to progress to the end.
Vindicated by History: For a very long time the original Zelda was highly overshadowed by A Link to the Past for being the first truly great game in the series for making the game much more focused and less cryptic. Come the Seventh Generation, with the explosion in popularity of open-world games, the first Zelda has now been acclaimed by many for being an early exemplary example of the Wide Open Sandbox and for letting the player discover things on his own. At E3 2014, Eiji Aonuma stated that The Legend Of Zelda Wii U will be taking inspiration from this game in crafting its open world setting.