This is my favorite game in the Zelda series and also one of the most enjoyable platformers out there. Like someone else said, it's a difficult game at the start and only gets harder the further into it you get. Death Mountain is everyone's least favorite part of the game, but I've always through it was pretty manageable.

I think that the main reason that most people don't like it is because it's not your standard Zelda type. A lot of Zelda fans now probably played the later Zelda titles, and Zelda 2 being the only game in the series with different gameplay isn't liked as much because of just that. It's not what they're used to playing as a Zelda title. I could be wrong, but that's probably the reason.