Movement speed, wounds and heal DEbuffs do not stack; at all. (heal buffs do stack)
Nothing stacks with itself. (6x Focused Mending is still 15%)
For everything else:
The last part seems to be exceptions, for skills that would be useless if they didn't stack, thus they were allowed to stack with everything.
Is there a list containing all of these intentional exceptions?
This seems to apply to buffs and debuffs in the same way. BUT:
What if a tactic changes an ability?
E.g. Big Brawlin' (BO), Focused Mending (KotBS), there are about a dozen of these tactics per side. Does this count as a tactic, since without it it would not have the effect OR does it count as ability, as it is still an ability that applies it?
Based on testing these tactics are split about 50/50, so there is no telling what is intentional, and it is really annoying to test all these by myself...
It would be really useful to have an "official" response, to allow us to make more meaningful bug reports. Because right now its anyone's guess what the intended exceptions are, and what the correct stacking rules are...
PS: Would be great to have something in the wiki about this. That can be updated if it changes, so we don't get the monthly 10 page question thread "Does a stack with b?".