It is really just an Adventure game, and if you say it's an RPG, than so is every other Adventure game.
Zelda is an RPG. RPG stands for ROLE PLAYING GAME. Taken literally, that means if you play a role, the game is an RPG. Of course, there are more qualifications. (mainly a complex story with an evolving character.) An RPG is essentially an interactive piece of literature. And, as many of the newer Zelda games have a deep(er) storyline than most other adventure games, I am compelled to place them in the RPG category. Also, Square pretty much MADE RPGs famous, and all new RPGs are judged against Square's formula. Sadly, many people (hopefully no one in these forums) think that any RPG without major stat functions, magic, an ATB or any other Square-esque battle function, and insane monsters is not worth playing. Still, I like Zelda, but it is Chrono all the way. GO SQUARE!