Tavis at the Mule Abides has been instigating a discussion about RPGs, particularly D&D, as a party game, by which he means making an RPG playable in short spurts of time, with little set-up, with the ability to add or drop players at a moment's notice. There's a follow-up post where he highlights some thoughtful analysis by James Nostack. I recommend reading it, since I have nothing directly to add to or expand on what's already been said about playing D&D in this way. I do, however, have a kind of tangent on the more general topic of off-the-cuff RPG design, since I actually tried to do this once.
The instigation was a 24 hour RPG design challenge several years ago; I had been thinking about making games more accessible to the general public, and one of the detracting factors is, I feel, the huge time investment. Ordinary people, as opposed to us geeks, want something quick and casual. So, my goal was a session duration of 10 to 15 minutes.
Ten to fifteen minutes. Including character creation and scenario set-up. No referee. No special equipment.
What I came up with was Comedy Blackout, a sketch-comedy RPG. It needs a lot of work, but I think the basic ideas are sound: use coins instead of dice, use scratch paper and pencil, characters are just an adjective and a stereotype, and the scenarios are basically sight gags. It's practically my version of Toon, and can certainly be played as a cartoon, but my inspiration was The Carol Burnett Show and Monty Python's Flying Circus, among other sketch comedy shows.
Eventually, I have to come back to this and get it working. For example, I need to think how to play the game without pencil and paper, something like "this salt shaker is the ATM, and this line of crumbs on the counter is the crosswalk". You need to be able to teach someone to play this at a bar or party, using whatever you have on hand. In fact, one of my goals is to condense and summarize the rules to the point that you can put them on a business card you keep in your wallet, so you can teach people the game; the game book explains the rules in more detail and provides options and ideas.
Also, there would be some standard scenarios based on the kind of party game non-roleplayers are familiar with: "If you were stranded on a desert island with only three things, what would they be?" or "What would you do if you won a million bucks in the lottery?" All the game would do is let you play the question out, instead of leaving it as a simple statement.
I bring this up not to toot my own horn -- the game still needs work, I think -- but to suggest this as a general goal for any newbie introduction to gaming. You need to be able to jump right in and start playing, and use whatever you have on hand, instead of special tools. We love our special dice, our character sheets, our esoteric books that delve in detail into our fetishized topics, and we're willing to devote a lot of time to studying and playing our games; but the average person you run into at a party or chat with during lunch at work is not going to feel the same way, at least not at first. They want instant, casual gratification.
Perhaps all an off-the-cuff D&D needs is a house-rule "0-level character, all ability scores are 10" and a way to make up quick scenarios. James mentions Tony L-B's Capes and the way you pick two "halves" to your character, then modify one element in each half. Perhaps you could do that in off-the-cuff D&D with a list of backgrounds (barbarian, mercenary, scholar, bandit) and a list of adjectives (cowardly, vicious, greedy;) pick one of each to make a character and give a +1 in situations where either half applies (+1 for cowardly characters to escape, +1 for vicious characters to attack, +1 for scholars to translate treasure maps.) Otherwise, it's still D&D, just a stripped-down version you can play without rulebooks.