So, some further ideas regarding this:
Player denisty dependent CC/condition could work very well as a balance for stacking, at least for Bleed- how would this work for poison, and burn, should be considered (more people = higher damage tick?).
Does Vulnerablity also need to stack higher? That could get broken at some point.
The downside to stability on CC for bosses is that people could just steal/reverse/etc the Stability, and render it useless; having defiant as a timed, counter-proofed Stability might be more balanced.
Alternately, Defiant also seems unique, if I’m not mistaken, in that it’s not a cooldown based skill like everything else in GW2 – once it starts it just never seems to end. So basically, you get one shot at hard CC In the beginning of the fight. What if Defiant had a cooldown, before it can start stacking again? Maybe add diminishing returns to the boss’s ability to stack it. That way, you have some extra chance to do something with CC skills.