I think that if Gauntlet was set, for instance, in New York during the 1980's and had you play as a team of guys with mullets, the argument as to whether or not it is an RPG would be the last thing on anyone's mind... Despite the play mechanics being exactly the same. Gauntlet is an action game that just-so-happens to have a cast strait out of the D&D motif.

And yes, Zelda is an adventure. However, you can have an adventure game with RPG elements, such as Castlevania: Symphony of the Night. Alucard can level up and get a whole succession of stat-changing weapons. Link can not do either of those things, unless you could the fact that his sword gets two or three modifications before the end. Zelda is strictly adventure.