Old school D&D's 2d6 reaction roll is a wonderful mechanic, elegant in its simplicity, and often underappreciated. It seems to me, though, that applying it straight to every encounter is a little uninteresting. Here's a quick system that I've cobbled together that expands on the 2d6 social mechanic by codifying the possible types of interactions and their effects on the reaction roll, and further modifying the possibilities according to a selection of NPC temperaments.
This system uses one quasi-new mechanic: An over/under adjustment which is either +1 or -1 depending on whether the roll is high or low. This tends to push reactions toward either a neutral or extreme reaction in certain cases.
The social interactions available to PCs are categorized as follows:
Converse: This is the default interaction, which encompasses such things as greetings, introductions, inconsequential talk, exchanges of information, simple requests, and anything that doesn't fit into any category below.
Reason: An attempt at cool-headed logical argument to persuade NPCs that your
point of view is correct and that it is in their interest to cooperate.
If the DM deems the argument sound and compelling, it's worth a +1 bonus to the reaction roll. Otherwise, treat it as...
An attempt to persuade the subject with an insincere argument based on
faulty but superficially plausible logic. The reaction roll is adjusted by -1
each if the subject's Intelligence or Wisdom score is 13 or higher, and +1 each for scores of 8 or lower (thus a maximum adjustment of -2 to +2.) A
intelligent and/or wise character is more likely to see the argument as
fallacious, while one with low scores is more likely to accept it as
Emotional Appeal: An attempt
to stir the emotions of the subject(s) through vivid language, moving stories, and impassioned pleading. On a roll greater than 7, the reaction is adjusted by +1. On a roll under 7, the adjustment is -1, as the listener is put off by the speaker's pathos.
something of value to the NPC. Reaction adjustment is +1 if the value
is on par with the recipient's wealth (i.e. the recipient could
conceivably obtain it through his own efforts without undue difficulty)
and +2 for something exceptionally generous (i.e. the recipient
could not otherwise obtain it, except possibly by spectacular good
fortune and/or personal sacrifice.) A pauper is a lot easier to impress with a gift than is a merchant prince, for example. If the reaction roll is still below 7, the subject is either insulted by the gift or suspicious of the giver's motives. Care must be taken that a Gift is
not perceived as a...
Bribe: Offering something
of value which is clearly intended as payment for the recipient's
compliance or cooperation in something he or she otherwise would be disinclined to do. Bribes are often considered insulting to honorable
characters. Lawful characters react at -2, Neutrals at -1. If accepting a bribe comes
with appreciable risk of harsh punishment, an additional -1 applies.
Offering praise to the subject, sometimes sincere but more often not.
Reactions are adjusted by +1 if the roll is over 7, and by -1 to a roll under 7 (which
indicates that the subject perceives the compliment as insincere.)
Intimidate: An attempt to threaten, bully, blackmail, or browbeat the subject into compliance
with the speaker's wishes. The reaction roll is adjusted as follows: +1 if the speaker is obviously in a position to make good on a threat (physically superior, better armed, in possession of sensitive information about the subject, etc.) +1 for every doubling of the number of the speaker's party over the subject's. +1 if the speaker has a reputation for ruthlessness. (Reverse adjustments in any case where the opposite is true.) On a positive reaction roll, the subject must check morale; failure
indicates that it will do as the speaker demands. Subjects who pass the morale check will be defiant and may take action against the speaker.
Entertain: An offer to share food, drink, music, or other forms of merriment. If the entertainment offered is a form the subject enjoys, reaction is at +1.
If the player's intention is unclear, the DM should either ask which interaction the character is attempting or make a judgment call based on how the interaction would appear to the subject.
Of course, how effective different forms of interaction are depends on the temperament of the NPC on whom they're used.
Roll 1d12 for the NPC's temperament:
Most temperaments are susceptible to a particular type of interaction. When subject to that interaction, they react with a +2 bonus. This supercedes any bonuses and conditions that normally apply to the interaction or to the NPC's temperament.
A Reserved character is not quick to judgment. He or she may be a deep thinker, even-tempered, dispassionate, or cautious. Reaction rolls are adjusted one point toward neutral; that is, rolls over 7 are penalized by -1, and rolls under 7 are adjusted upward by +1. Reserved characters susceptible to Reason.
An Irascible character is irritable, surly, or ill-tempered. All reaction rolls are penalized by -1. Irascible NPCs are not especially susceptible to any interaction. They're a hard lot to please.
Agreeable characters are affable and eager to please, sometimes naively so. They are not particularly susceptible to any interaction, but all their reaction rolls are adjusted by +1 as long as the speaker is not abusive or threatening.
Passionate characters are enthusiastic and fiery; they love and hate with gusto and are seldom apathetic. All reaction rolls are adjusted away from neutral: rolls over 7 gain a +1 bonus, while rolls under 7 suffer a -1 penalty. Passionate characters are susceptible to Emotional Appeals.
"Normal" characters have no particularly strong temperamental tendencies. Reaction rolls are made without adjustment. Hey, got to draw the baseline somewhere.
Submissive characters are meek, timid, subservient, or weak-willed. Submissives are susceptible to Intimidation (and not subject to the morale check rule; a Submissive character genuinely wishes to appease the speaker.)
Vain characters have an inflated sense of their worth or importance, either in general or with regard to appearance, talents, or status. They are susceptible to Flattery if it is targeted at the source of their pride.
Jovial characters are fun-loving, and susceptible to Entertainment, if the form offered is agreeable to them.
Acquisitive characters are driven to obtain things, either material, such as money and goods, or intangible, such as knowledge or power. They are susceptible to Gifts and Bribes of appropriate things.
These temperaments aren't necessarily mutually exclusive, so it's theoretically possible to have more than one apply to the same character, e.g. someone who is both Passionate and Vain, or Reserved and Acquisitive, but that's probably more complexity than the game really needs. I wouldn't recommend using more than one per character except possibly for very important (non-random) NPCs.
Over/under adjustments, such as for the Reserved and Passionate temperaments and the Flatter interaction, are applied after any static adjustments, including Charisma and interaction bonuses or penalties. If both the interaction and the temperament specify an over/under adjustment, then both are applied simultaneously (again, after static adjustments.) Thus, an Emotional Appeal used on a Reserved character cancels out, while Flattering a Passionate character results in stacking bonuses or penalties.
For example, a player character with Charisma 16 seeks to gain the cooperation of a Passionate NPC. The speaker, not knowing the NPC's temperament, chooses ordinary conversation as the mode of interaction. The reaction is rolled, adding +1 for the speaker's Charisma. Thus, the threshold for the over/under adjustment for the Passionate temperament is 6 instead of 7. If the speaker had offered an appropriate gift, it would have been 5.