As a 12 yr old boy, this game was a huge disappointment for me. It wasn't a failure, because I played all the way through it .. and had some fun moments. But, at the time, so much more was expected from Zelda, and it failed to deliver. That is why it gets a bad rap.
It would be like Bungie releasing an above-average Light Gun (or RTS) game for Halo2. It would still be Halo2 and it would be a solid game .. but you bought the game wanting a Halo style game.
My biggest gripes for it back then was that the Overworld was hideous looking. The townspeople were basically signposts (no interaction) .. and then the Dungeon levels felt like uninspired Metroid.
After about 3 dungeon levels, you figure out to level yourself up before the next one, and it makes the game pretty easy. Zelda 1 took me 3 weeks of playing to beat it ... Zelda II took about a week. I didn't feel like I got my money back.
The evidence that Zelda II was a failure, was that Nintendo NEVER tried that playstyle again. Although, I agree with a previous poster that OoT shares more with Zelda 2 then it does Zelda 1. It's like Nintendo had a vision of how they wanted Zelda to be .. but the constraints of 2D didn't allow them to achieve this. So they had to wait.
I'm not bashing .. it is a solid game, and better than 75% of all the other NES games at that time. But a higher level was expected.