I've given a lot of thought to modeling social interaction and studied how many games have done it. Some of the best examples out there are Burning Wheel (Duel of Wits) and REIGN. I'd like to have a fairly open discussion about how to model social interactions in RPGs beyond the one-simple-die-roll treatment most systems give them ("Make a Diplomacy check"). Generally off-topic here are "non-starter" directions like, "Don't do it in the first place" or "Punt and model story instead of actual interactions." I already know how to do those things, by and large.
Social systems in real life are complex networks of people. If you piss off Joe, then Joe will tell his best friend Hera you're an asshole, and Hera's reactions to you will change as a result. A one-check system like D&D 3E/4E's Diplomacy check models only a single interaction between a PC and an NPC. Everything else is left to GM fiat. It lacks rules for the larger system, including trust networks, reputation, propaganda, mass-effect speeches, and other society-affecting actions.
Social systems in real life are generally not like combat. A lot of game systems try to give social interaction the combat treatment, and that's a huge mistake. Unless you're a sociopath (and, hey, maybe most D&D characters are), you don't go around treating everyone like an obstacle to be crushed into submission or manipulated/deceived into doing what you want. Healthy people build trust, negotiate to get what they want, and generally treat others well. Treating people badly (manipulation, deceit, intimidation, and the like) are choices that players can make in the game, but they should not be the only tools available. D&D 3E/4E gives a nod to this; it offers "kind" Diplomacy as well as "mean" skills (Bluff and Intimidate, mainly).
Social systems probably should not follow the back-and-forth "exchange blows" techniques used to model combat, either. Turn-taking is probably unavoidable, as this is a game, but I definitely want to shy away from anything that feels like "roll initiative; roll to hit; roll damage."
I want a system of medium complexity. I want there to be some currency that gets moved around and "gamed." It should be around the same complexity as Basic D&D's basic combat system, or simpler.
Here's what I'd like a player to be able to do.
Build (or lose) trust. Make friends and know they'll be there for you, and test those friendships when things get rough. Earn the trust of groups of people, not just individuals. Morally bankrupt characters can choose to fake it, if they want, but trust is generally earned (tested over time). When you betray a person's or group's trust, you should lose their trust. If you burn people badly enough, they turn into enemies.
Counteract (or spread) propaganda and lies. Controlling the message is an important part of politics. A good social system lets you change and protect belief systems.
Gather (or obfuscate) useful information. Spies deal in information. They collect it personally but also use other sources. I want a system that treats useful information as a game currency, so that it's more than GM fiat and/or fictional positioning. Most games let you figure out that the King is secretly sleeping with the Duchess, but few games give you tools that help you do anything with that outside of fictional positioning. That information should be usable as leverage in some kind of social interaction, whether to earn an ally's trust, intimidate or threaten someone in the King's or Duchess's court, or spread rumors that erode public trust of the King or Duchess.
Negotiate. So much negotiation in RPGs is left to player skill. It comes down to the player making a convincing argument. I'm not saying that's a BadWrong way to play; it's just not what I'm looking for here. I want a negotiation to be a combination of a little player skill and a lot of character skill (and utilizing a fair amount of game currency). The Duchess's footman trusts you and owes you a favor for helping him when he broke that china plate, so he's probably willing (+5) to let you into the manor house and not say anything to anyone, provided you leave his name out of things. However, consider things even. Maybe his trust for you isn't so carved in stone, now that you pressed him for this favor, and if you get caught and people ask him, he might turn on you (-2) about some other issue. Or you've built a good relationship with the local smithy, buying from him all the time at fair prices, so he doesn't mind cutting you a deal on that new armor, at least this time (all handled with +/- modifiers and scores and such).
Probably other stuff, too. That's off the top of my head.
What kinds of currency do I need? What kinds of systems/actions/player interface? Can one system handle groups as well as individuals? If so, how do individuals in groups affect their groups and vice versa.
How can I avoid a cynical management of trust as sociopathic currency?