First, I agree wholeheartedly with those who say that a detailed social world (that the players understand) is a must for a roleplaying-intensive adventure. I try to work with the players to flesh out background--and this will inlcude giving them an understanding of what they know, how others react to them, and how various situations work out.
I also use several mechanics to level the table (so to speak) among those players who have extroverted, dynamic personalities and those who do not. First, I allow flexible skill checks to modify NPC behavior (loosely according to the chart in the DMG) with bonuses (or penalties) based in player interaction. For example, the party was dealing with the leader of a clan of lizard men, trying to defend a member of the clan from charges of disloyalty and incompetence (charges that were partially true). They have had few dealings with this culture (-2). They use their aristocrat as a spokesman (reputation check (using system from Wheel of Time RPG) works, +2). The wizard, true to character, contributes by spelling out why it would be in the leader's political interests to not punish the person in question (good argument by player, +2). The group promises to help in the future, punctuated by a threatening display from the figher (successful intimidate +2). The aristocrat's player makes his speech, which I judge for effectiveness based on my knowledge of the NPC's agenda, taste, and interests (in this case, merely passable--0). Only after all of these players have had a chance to influence the outcome (circumventing the "netrunner" problem), and actual roleplaying has been allowed to occur and affect the situation, does a skill check come: a diplomacy roll, modified as above, which barely works.
Short answer: a flexible approach to skill checks and aid by others in the party can provide a mechanic that encourages players to select knowledge and social skills for players, fights the tendency to over-specialize characters, and mitigates the imbalance between players of different personality types. And all of this is done within d20 mechanics.
I also use several mechanics to level the table (so to speak) among those players who have extroverted, dynamic personalities and those who do not. First, I allow flexible skill checks to modify NPC behavior (loosely according to the chart in the DMG) with bonuses (or penalties) based in player interaction. For example, the party was dealing with the leader of a clan of lizard men, trying to defend a member of the clan from charges of disloyalty and incompetence (charges that were partially true). They have had few dealings with this culture (-2). They use their aristocrat as a spokesman (reputation check (using system from Wheel of Time RPG) works, +2). The wizard, true to character, contributes by spelling out why it would be in the leader's political interests to not punish the person in question (good argument by player, +2). The group promises to help in the future, punctuated by a threatening display from the figher (successful intimidate +2). The aristocrat's player makes his speech, which I judge for effectiveness based on my knowledge of the NPC's agenda, taste, and interests (in this case, merely passable--0). Only after all of these players have had a chance to influence the outcome (circumventing the "netrunner" problem), and actual roleplaying has been allowed to occur and affect the situation, does a skill check come: a diplomacy roll, modified as above, which barely works.
Short answer: a flexible approach to skill checks and aid by others in the party can provide a mechanic that encourages players to select knowledge and social skills for players, fights the tendency to over-specialize characters, and mitigates the imbalance between players of different personality types. And all of this is done within d20 mechanics.