When am I supposed to do anything? If I ever attack, it just gets blocked. But the enemy can just stand there and heavy attack over and over and I can't seem to block and counter with anything.
I don't have any actual data, but just from feel it seems like heavy attacks are advantage on block, lights are disadvantage on block.
Heavies are much slower so more susceptible to being interrupted by lights, dodged, or parried (press heavy attack in the same direction when the attack flashes right as it hits). Parry seems to give a free hit.
Heavy attacks also take a lot of stamina from what I remember, and if they become fatigued by running out of stamina, they become sluggish in all actions and are knocked down by a lot of stuff. Fatigue knockdown seems guarantee a free heavy.
As you can imagine, there's a ton of character specific things as well that come in to play that change the dynamic. For example, berserker has an infinite chain of attacks as long as he has stamina.
Also it's worth noting, there isn't much guaranteed damage as far as combos and whatnot. All that is character specific. A chain listed in move list is not a natural combo (opponent can block the followup when hit)