For me the prohibitive difficulty of Zelda 2 was the Swamp Temple. Or rather, it was reaching the Swamp Temple. I did level farming for some 102 continues before I could breach the walls of that fortress.

After that (and after resorting to a guide) the rest of the game was fairly easy. Part of the difficulty comes from a blind run: if you don't know where to go and what to look for you lose a lot of energy on random encounters and battle you should really be skipping.
I still have not beaten the game, though, even after years of trying. I made it to Thunderbird in the Great Palace repeatedly but could not defeat the boss. Since there's only one fight after that (Dark Link) I decided I was close enough.


Still, I agree that the difficulty wall is an opinion (even though it is a popular one). There are certainly other contemporary games (like the NES Ninja Turtles game) which make Zelda 2 look like a walk in the park.

EDIT: Well, the first Zelda game after Zelda 1 to have a second quest was Ocarina of Time, and that only came when it was ported to a new system (the Gamecube if I recall correctly). I will admit, though, that OoT Master Quest is of a caliber of gameplay on par or exceeding 2nd quest; the dungeons were clever and forced you to think to navigate rooms (such as transporting torch flames through doors), a notion not frequently explored in the 3d Zelda games.