I thought this would be an interesting topic to discuss. I hope we can talk about it without hard feelings. Not all of us have the same view of what a roleplaying game is and I would like to see what ideas are out there. I hope we can use this as a chance to learn and teach and explore some of the more obscure parts of the hobby.

Defining what an RPG is in its simplest form: Is it a game where a player takes on a role (aka character) in a setting run by another player (aka gm). I don't think I could define it any other way. It seems to be the only commonality among all of them.

D&D (d20) is the most obvious one and the that set the standard. But what about Everyone is John does that count as a RPG? Is a voice a character? I love that game and it is fun to play and definitely twists the concepts of a setting, John's head, and a character, a voice in john's head.

Does it have to have a random dice mechanic or just any resolution mechanic? Pathfinder uses dice, but Amber is completely diceless. Fate has unique dice or card sets. Hell D&D introduced unique dice to the world of gaming. Before that a d6 was what most of us grew up on.

Do RPGs have to have a progression system? Are growing, improving characters a staple of an RPG? Personally, I don't think that is necessary for a game to be a RPG. It is something that helps longer RPGs to be interesting and to give the players something to work toward.

Levels, XP, fate points, gold, credits, aka loot is just a reward system. Do RPGs need to have reward systems? or is it just a technique to get players to play the game the way the designer envisions?

Is something like Gloomhaven an RPG? You play a role, get rewards and grow. The only difference between an RPG in book form and Gloomhaven is the amount of tokens and props used to play it. I still think it works as an RPG.

Kingdom Death seems like Microscope or the quiet year as you go in and out of different "resolution levels" of each game. In Kingdom Death you play survivors or the the entire camp of survivors. The Quiet Year you jump into individual roles as they are added to the world. Microscope works at an even higher resolution where you jump back and forth in time as well as places and characters. All of these have temporary characters. That is the player isn't tied to any one character. Those characters may enter and leave but the player continues playing. Does a RPG require having a continuous role for the entire game/session/adventure?

Kingdom Death is a bit unique to me because it adds a mechanic to simulate a game master without needing to have anyone play the actual role. I'm sure there are other games with the same kind gm-less systems built in but none come to mind. I know there are books you can pick up to run D&D RPG GM-less. So are they still considered RPGs or is the definition more like: Is it a game where a player takes on a role (aka character) in a setting run by a player or mechanic so that it is dynamic? So those are my thoughts and questions that got me thinking about the subject.

EDIT

I thought I would expand on this as my question became clearer as I tried to respond to everyone. What do you expect from a roleplaying game? I think that is the real question and is much easier to answer than defining the amorphous label of RPG.