I have made my thoughts about this question too, and I think there are three points which make a rpg a rpg.
first, it is the battle system. This is more complex than in adventures. In adventures, you have to find the weak point of your opponent and then hit him between 3 and 9 times. In a rpg, the opponent has a number of live points which have to be taken away from him. This is done with a lot of different attacks, and depending on the opponent some are stronger and some are weaker.
second, you can level your character, and he has livepoints and not hearts or honeycombs or something like that.
third, and in my eyes most important for the gameplay of an rpg, there is the story. but the difference does not lie in the overall story, because a lot of rpg's don't have more interesting stories than zelda or metroid or even mario. the difference is how the story develops. in zelda, somebody tells you that you have to get three (or five, or seven) items, and then you go to dungeons to get them. in rpgs, you follow a goal, but on your way you will visit many cities where the people have problems and you have to help them. this will not only help those people but also you.
