I don't know what it is that technically makes an RPG an RPG. However, I think that I know one when I see one, if that makes any sense. I go by the general feel I get from them. A few games that, to me, are "true" RPG's:

-Final Fantasy (well duh)
-Tales of series
-Star Ocean series

Games that I feel blur the lines somewhat, and that I wouldn't class as "true" RPG's:

-Kingdom Hearts
-Zelda

Before someone gets up in arms about the nerve of me using the term "true RPG", I do understand that the only "true" RPG's are probably games like D&D. To me however, I loosely base my preconceptions on what an RPG should be, rightly or wrongly, off Final Fantasy. And those are the type of games that come to mind when I hear the term "RPG"