Whipper: I think it would be cool if we could see marks on boss when a mellee character slice him with a sword or an axe etc....
We can see that the boss have sword marks and axe marks or that he look tired and he walk slower etc. till he die... I think it could add some realistic aspect to the game and more fun killing bosses
Bashiok: For boss fights we don't really want to just start throwing "damage decals" on them. It can very easily be too subtle if you're fighting a smaller boss or one that has odd geometry to it, and it can also end up that no matter the flavor, style, and cool look to a boss you just end up fighting a big gory mess.
Also we think that we can go a lot further than just simple decals and get something much cooler out of a boss fight. So what we hope to do for at least the bosses where it makes sense is to show more apparent and visually noticeable damage states. Instead of a blood splatter they may actually lose a limb, or a piece or armor, or - like the Thousand Pounder - may transform in some dramatic way changing how the fight progresses. We want to go a lot further than just showing sword slashes and instead use dramatic and noticeable (which can be difficult in a game like this) ways to indicate a boss' current state.
Speaking of battle damage though, and I don't think this came across in the gameplay video or any of the screenshots as of yet, but when an enemy dies a critical death it actually drenches your hero in blood if you're within proximity of the spray. It's awesome.