I never could beat Zelda II without the assistance of Game Genie. And even then, it was (as @SUCCESSOR said), hard as shit. And if your shit is as hard as that game, you need to drink more fluids.

One other thing (not what I would consider a flaw, though) was the absence of a second quest. That was one of the things that made the first game so awesome. It was almost like two games in one. Come to think of it, I don't think any of the later Zelda games had a true second quest--it least, not one of the caliber of the original.