I think the whole idea of genre distinctions should really just serve the matter of rough orientation. I've read pages and pages of flamewars concerning whether Zelda games are RPGs or not and similiar stuff. And you know what: It doesn't matter. I mean, sometimes I'm not even sure what video games are.

This has been debated to death many times. The answer is almost always different depending on who you were to ask. To be truthful, there is no such thing as an RPG if you get right down to the nuts and bolts of the issue, much like how you spoke of playing roles as something you do in every game. As it's my favorite genre however, I'll go ahead and share the first things that come to mind when I think of an RPG (in order).

1. Story. Most all games have a story to tell, but the stories in RPGs tend be be the save the world from some great evil type. As a rule, you always start weak and then improve your ablities as the tale unfolds to reach the conclusion, which leads me to my...

2. Stats. It is not an RPG if it doesn't have some sort of stat to increase. It doesn't have to involve skill points or be as involved as something like Oblivion and it's attributes in things like reading, but there must be some tangible numerical ('numerical' is important here) way for a player to realize they have improved or leveled.

3. Exploration. Now, different RPGs handle this in there own ways. Some games allow overworld exploration while some only do the point to point thing (which I think is boring) with dungeons (or some other field of exploration). If you aren't on a mission, then what's the point of playing?

4. Dungeons. It doesn't have to be an actual dungeon mind you, but some cave, sewer, castle of some sort to explore is necessary.

5. Monsters. It can be Pokemon, human characters like Sephiroth, a slime from DQ or even mythological/biblical demons... There have got to be monsters of some sort in the game with which to do battle. Speaking of battles...

6. Battles. They can be tradition turn-based (my favorite) or the more modern action systems, but most all RPGs break down into one of these two groups. Aside from some hybrids like Mass Effect you don't see a lot of FPS shooting in RPGs. You also don't see action game stuff like turtle shell jumping and the like.

So... What have I said in all of this? RPG, to me, is just a classification of games that brings these thoughts to my mind. That's all.

http://en.wikipedia.org/wiki/Role_playing_game

RPGs are the games that borrow mechanics from older, pen and paper rpgs, IE: Baldur's gate/Neverwinter Nights, and to a lesser extent jrpgs (there's no movement about the field, so they're jrpgs). This debate of 'what is an rpg' has gone on forever, and no, it's not a game where you 'play a role'. That's just silly, as every game under the sun would be an rpg.

RPGs, to me, are the games that are generally turn based, give you the ability to choose how your characters grow (stat wise), and have a erlatively open world (you can go wherever you want whenever you want).

To me, an rpg is a game where the focus is on playing a role (lol). By that i mean the focus is on your characters, not the gameplay. For most games, the focus is on the gameplay, like fps games, puzzle games, etc. In an rpg, the real meat of the game is in developing your character(s); both in the story, and in power (levels, stats etc).

Thats why jrpg's are rpg's, why the diablo series are rpg's, why games like Bioshock are considered to have rpg elements; all focus on the making your character stronger/better as you advance through the story.

Blehman, yet again, takes the cake

Also, a game where you upgrade you character over time.

Oh that's easy. That guy's an asshole.