There's an incredibly fun groundwork to the original Zelda's mechanics. The combat's focus on position and blocking feels engaging, though it is incredibly challenging. The various items are useful and pretty quirky sometimes. It's funny to imagine Link carrying a bridge around the whole time. The game's visual aesthetic is also kinda utilitarian with some fantasy stylization to it, the color signifier for enemy difficulty is pretty cool since it applies to Link's armor as well, giving some nice consistency to the world.
That being said, The Legend of Zelda is a game I wish I could play blind, allowing myself to explore. Yet I find myself dissatisfied with the obscure nature of its progression and secrets. The openness of this game is greatly beloved (especially in the advent of BOTW) but there's little in the way of design to actually telegraph the things the game expects you to do. A lot of my time with the game was just regret that I had to check a guide to progress.
It's hard to justify critique on a game like this given its historical context, and it is definitely revolutionary for its time, but I think the first Zelda game just does not appeal to me in the way that its successors do.