KidSnide
Adventurer
I think what you have there is a puzzle system. You have to guess or attempt to figure out where the guy's weaknesses and strengths are and then "push the buttons" to overcome them. How do you determine what the NPC's strengths and weaknesses are? That strikes me as the most important thing in this system. (Aside from pre-play character building.) Though on second thought the modifiers (+/- 2) might mean that sort of information gathering is rather pointless.
The skill challenge variant discussed upthread is designed to address nearly all of these issues. +/- 2 modifiers are replaced by +/- 15, +/- 10, and +/- 5 modifiers, and a substantial part of the challenge is figuring out the NPC's strengths and weaknesses, either through reasoning or through Insight/History/Streetwise/etc checks.
How do you determine what a successful action means? Especially if no one is going to describe what sort of interaction just took place.
At what point do you call for a roll? How do the players trigger one? How does the DM judge that a roll is called for? Especially if you don't want anyone to have to describe what actions they are taking.
Is there any way for the NPCs to "push back" against the PCs?
I view a successful role as the NPC accepting that the PC made a good point, but not being convinced yet. (I tend to nod and say something explicit like "That's a good point.")
I let the PCs talk (either role-playing what they say or just describing the type of argument they make) and call for a role when they are done. I give a small modifier (usually +/-2, occasionally higher) for good role-playing, but mostly I pay attention to whether they have used the evidence that is key in the conversation or played on a NPC weakness.
After everyone has a turn (either gathering info or making a persuasion role), I have the NPC push back against one specific PC, usually forcing a role on a topic the PCs would prefer to avoid.
Is there a way for the DM to judge what an appropriate modifier is? Actually, more to the point, is there a way for the players to judge what an appropriate modifier is? That way they will have some sort of way to judge which action is better than another (ie. meaningful choice).
The players know that a core point or major weakness is worth a +10 modifier, with a +5 modifier for a supporting point or minor weakness and a +15 or +20 for a killer point / auto-success. That usually means that it's always worth making an argument if you think you have a good point, but only the most persuasive characters should try prevaricating with nothing.
Still, it's a good point. I'm not sure the players get as much information as they should. Maybe a good roll for info gathering should tell you how good a point it is?
-KS