When I was little I HATED this game. Mainly because it was hard, and I couldn't get passed the beach for more than two minutes. I later played Ocarina of Time thinking I might hate it, and then it ended up being my favorite game of all time.
I later went back and finally got to play it again thanks to Action Replay and a copy of Animal Crossing for Gamecube. Besides being stuck with a GC pad, I had a really awesome time, now that I was a more experienced and smarter gamer than I was when I was 3.
For an NES game, the first Zelda is HUGE. Not so much compared to later games, but back in the day there was a reason why the game had one of the first batteries to save your progress. The graphics have aged yes, but I think they maintain a certain charm that still looks good in my eyes.
It may not be the best game in the series, but it laid out a great framework for the rest of the series. Though you might need a internet guide (some of the puzzles and secrets are a bit bullshit, I think there is one dungeon that's under a random bush) to finish the game, it's quite good not in terms of just length but quality as well. A must for any Zelda fan, and back then it really showed the world that Miyamoto could do something amazing besides Mario.