As to the armor rating, a dragon would become very powerful, but I don't have a problem with that actually, perhaps the people in b.a.d.d. would even applaud the idea. Regardless, it works both ways, a character class with the best defense bonus is also going to be a lot harder to hit, so I would think that it would kind of neutralize itself out.
I was trying to decide whether or not to let it stack with armor. I had a thought that that maximum armor bonus a character could have and still gain the full use of their defense bonus would be equal to their strength + dexterity modifier. So by that I mean if a character had 18 strength and 18 dexterity, they could wear any combination of armor that was up to +8 to AC, that being non-magical, magical bonuses to armor would not affect the total. If a character were to wear armor that granted a bonus of +9 or greater they would lose their defense bonus, because the armor impedes their ability to move freely while in combat.
As to action dice I was pondering a few ideas about advancement of dice. I was thinking sorcerers, barbarians, and monks, who all basically use their iunner self as the focus for their abilities would get the best action dice progression, I realize in spycraft it is by level, but I was thinking of doing it by class. Druids, rangers, paladins, and bards would have the second best progression, and classes that are based fundamentally on training, meaning fighters, rogues, wizards and clerics would have the poorest progression.
Just a few more thoughts.