So Ben Lehman is poking fun at Rich Forest's tongue-in-cheek definition of a roleplaying game which is, simply "A game that resembles D&D." Which is all very funny and ha-ha. It has been dubbed (by Nir Shiffer) the Forest Criterion, which unfortunately means that I am almost incapable of identifying roleplaying games, having had nearly zero experience with D&D. Them's, apparently, the breaks.
More seriously, I've never quite understood why we have such trouble defining a term which is self-definitive. A roleplaying game is... get this... a game where we play roles. It's like word games, which are games where you play with words, or math games where you play with math, or real estate games where you play with real estate.
Identifying whether or not something is a roleplaying game pretty much comes down to two yes-or-no questions. First, is it a game? Second, do the players play roles? If both answers are yes, then it's a roleplaying game. If you only net one "yes" between the two, well then, it's either a different kind of game or a roleplaying experience-thingy. But you have to have both elements in order for it to be a roleplaying game.
Dungeons&Dragons - it's a game, there are roles that you play. Yes, it is a roleplaying game.
Dogs in the Vineyard - it's a game, there are roles you can play. Yes, it is a roleplaying game.
Once Upon A Time - it's a game, but you do not play roles. It is, in fact, a card game.
Sensitivity Training At Work - it's not a game, but you do play roles. It is, in fact, a roleplaying experience-thingy.
A Ripe Banana - it is neither a game, nor does it have roles to play. It is, in fact, a fruit.
I mean, seriously, this isn't moon-language, is it? Roleplaying game -- a game in which you play roles. What's the difficulty?