While I find all role-playing games to be an exercise in collaborative storytelling, I think that PBeMs take this to a greater
extreme than face-to-face games.
With FtF games, a group of people sit around a table, roll dice, tell bad puns, and participate in the action of the story pretty much in real time. Short
descriptive phrases, hand gestures and funny faces set the scene and help each player visualize the action or activity.
PBeM games, by definition, are pure storytelling. They require that the players have some writing skills to describe each action or gesture. They require the players to be willing to trust one
another and play off each other more than FtF games, because the interaction is through words alone without the immediacy of voice, tone, or gesture. (Unless, of course, the writer is really