The green health bars right to each portrait are pretty self-explanatory. But Cassandra has some grey symbols above the health bar as well:
I've seen those disappear and come back while playing, but I have not yet been able to find any explanation about them. What exactly do those grey bars represent?
This is Guard. Warriors have the ability to give themselves extra damage soak, like temporary hit points. These HP are lost before real hp, and it functions much like the mage barrier spell, but is generally only for the warrior. Although it absorbs less damage than barrier, it can be gained from several abilities, and it stacks with itself.
This is part of the removal of healing spells, and there is a pretty good write up about it.
