Getting a critical hit on a Parley roll just seems... weird, y'know?
All due respect Scrapey my man, but no... I think
you are the weird one! This other time I was playing D&D, our party was hanging out with these two tengu, and at the end of the session, after we killed some monsters and looted some treasure together, the DM asked us to make reaction rolls to see what the two tengu thought of us. I rolled a crit! So they wanted to be my 5th level magic-user's henchman (she is a 12-year-old girl, btw). Technically, I had a +1 Charisma bonus, so an 11 would have had the same effect, but I still remember that roll. It was exactly like having manipulate opened and rolling a 12+ in AW -- they became my allies.
Of course, just like them Cloud Giants thing up there, I remember it because I got two awesome bird-man retainers, but "rolling a 20" is shorthand for getting a cool thing that happens only rarely, and only because of luck, so there's that I guess.
Going back to Adam's "turning the tide" thing, though... When I learned about the tremulus rule, I thought it would be better if everybody was affected because it focuses attention back to the table and the other players. When somebody rolls and it has nothing to do with your character, you don't have a lot of reason to care. But of there's even a small chance it might affect you (in a good way), there's incentive there to at least pay enough attention to check the roll, which has an effect on your attention as a whole. I think your "natural 12 effect" should probably be different for each character, though. Your guy always gets the same bonus thing, whatever it is, when anybody rolls boxcars. My guy gets something different. Maybe the cleric can heal himself or somebody else right away, the fighter gets an extra damage die to use later on, the mule gets an extra piece of gear stock, etc etc.
As opposed to like a thing that the GM says that is good for everybody, because that doesn't focus their attention onto the person rolling, it's just the GM reminding them that a thing happened.