I've been playing it recently and I've really been feeling how much the modern Elder Scrolls games have in common with it.
You've got this open wilderness that you can traverse more or less freely, tons of stuff you can do before tackling any dungeons, which you can take in more or less any order. Lots of random secrets you can find basically anytime you want. Dungeons hold items, but most of the time they're unrelated to the completion of the dungeon itself, and you could easily go on without the majority of them. You even have a minor NPC-based sidequest, when you bring the letter to the old woman to buy potions. One thing it lacks is dungeons unrelated to the main campaign progression, but I see that as due to technical limitations more than anything else. They didn't have room on the cartridge for small side-dungeons, so they just made them all one-room caves.
If Nintendo had decided to follow up the original Zelda with a focus on the open-ended overworld exploration aspects, rather than on the dungeon-based story progression, by today you'd have a series that looks a heck of a lot like Elder Scrolls. And if you tried to go back and make an early NES version of Elder Scrolls, you'd come out with something looking a heck of a lot like the Legend of Zelda.
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Black Turtle spilled my diet soda!