A pioneer in the industry
I find it pretty great how balance this game is compared to others that were released around this time. When you play games from other companies, I find it a little frustrating at how clumsy the balance of those games are. Games like Castlevania or Ninja Gaiden, while still good games, felt unbalanced at how difficult it is to stay alive. On the other hand, I feel like Zelda makes you feel more in control of that factor.
This is accomplished through the exploration aspect of the game. Unlike other games before it, where if you couldn't finish a level you just had to try and try again until you finally got it right. But in Zelda, if you can't finish a level you can go back to the overworld and possibly find a way to extend your life, or buy a better shield in order to return to the level and try again, but this time some factor of that level has changed which makes the game less frustrating to play though.
As I can't be totally sure of this since I haven't played every single game from around that time period, but all the games I played before that were based on "make it through the level" and your difficulty factors were either a time limit, killing a certain number of enemies, or finding keys. (which admittedly Zelda also does but the inclusion of Zelda's item system makes it more interesting.)
I will agree with some other negative reviews of this game that the graphics were very simple even for it's time, but I feel like this was done on purpose. The same reason that there wasn't a town with villagers that you could talk to. The fields were a solid bland color, all the dungeons are basically monochromatic, even the dungeon BGM is very repetitive. I get the feeling that these were mechanisms used to create the feeling of loneliness. Sure, there are some people in the game that sell you items, give you money, tell you a hint, or gamble with you, but the text is so sparse and robotic (again possibly on purpose) that it accurately conveys this sense of loneliness never felt in game worlds before.
Some rooms in the later dungeons seem to only rely on throwing more baddies in there as a difficulty factor as opposed to finding a smarter way to create challenge. I also hate the way that they re-use bosses from earlier dungeons and just throw them into the later dungeons. Seems like they just wanted to get the game out so they started to recycle old content to push it out quicker.
The Bottom Line
Zelda is a game about accomplishment. Much like the early Metroid games, it's fun to play through and find an inaccessible area, find an item meant to reach that area, and then it clicks in your head. You really feel a sense of accomplishment when this happens because even though it is a video game and was definitely a pre-determined scenario (by the game designer), you feel much more like you found this item and thought up a really cool use for it as opposed to using it for more firepower or more life. That is really what made this game unique and why it was a pioneer for the industry.