I love the original Legend of Zelda. It's a game I've been playing for years and years since I was five years old, and to this day it's never gotten old or boring or easy. Especially the second quest. I love the fact that there's a whole new quest after the first. That was like getting two games in one.
And I kind of liked the fact that you could only hold 255 rupies. It really made the game more challenging, especially when you really needed medicine or the big shield or some other item. It's nice to actually need rupies, instead of having more rupies than I know what to do with like in some other Zelda games.
I like the different areas of the map and I love the exploration aspect of the game. Nothing was really handed to you the first time playing. The desert, mountains, lost woods, forest, were all nice different areas to explore. Bombing random walls/rocks and setting fire to different bushes to find secrets was really fun too.
The bosses are challenging, yet not too difficult. Even though I've played the game countless times, I still die every so often trying to kill a boss. Even the enemies. Wizzrobes are some of the toughest enemies in the game. They still kick my butt every time I play. Darknuts are so much fun to kill, trying to sneak up on them from the side or from behind was a great challenge.
And I love the non-linearity. You can go back into a dungeon as many times as you want/need to, get heart containers whenever you want, and do most of the dungeons in any order (except Level 9, of course). I usually do Levels 7 and 8 before Level 6. I save Level 6 for last because of the enemies in there. I always want to have the most heart containers possible before doing Level 6. Another thing I do sometimes is do Level 8 as soon as possible to get the Master Key. I try it just for a big challenge, even just after beating Level 1, when I only have just a few hearts. Then I don't have to worry about keys for the rest of the game.
So yes, I love the game. I don't think I'll ever get sick of it. I still haven't mastered the Second Quest, and it'll probably be years before I do.