First: Creatures have an extraordinarily difficult time hurting each other successfully. It seems that multiple successive blows aren't much/any more harmful than just a single blow. A high-skilled dwarf with adamantine equipment can consistently fracture a bronze colossus' skull, over and over again, yet the skull will never break, and bite attacks that simply tear a creature's muscle over and over don't do much either, it seems. I have an iron man stabbing an ettin with a pike right now, and is good enough at it to fracture the ettin's skull multiple times, and the ettin's wound page is almost entirely red/cyan, yet nothing is happening.
Successive attacks should matter more. After all, if you can tear skin, you can keep tearing skin until you get at what's underneath, and if you can fracture a skull, you can fracture it until there's not much left, and also get at what's underneath there (if there is anything). Wounds essentially seem to be limited to what an individual attack can cause, here, which is odd. A bronze colossus should not have to die from a single skull fracture, but it also shouldn't be able to accrue eighty of them and still be as affected as if he had accrued one or two.
Death from blood loss doesn't seem to happen a lot either, at least not from regular wounds. From magma, sure, but not from constant bite wounds and pikes-through-the-groin.
As far as creatures damaging each other is concerned... try putting, say, two hydra against each other. Basically nothing will come of it; they'll fight for ages and never kill each other.
For what it's worth, some bronze colossus body parts also don't drop as the body part, instead being called "bronze colossus's bronze" or something like that, as if it's just a bronze tissue chunk.
Also: Temperature transfer also seems messed up. You can dump lava right on top of an elephant and it can walk away wounded but alive. Creatures survive in magma for far too long. While they should die almost instantly in most cases, I've actually had creatures drown before getting burnt to death. Also, they seem to either die of melting or blood loss, which is strange, since they should mostly burn to death instead. Magma also seems to be unable to set corpses/remains/bodyparts on the ground on fire, except maybe after an extraordinarily long time. This is highly opposed to what should happen, which is severe burn wounds and almost instantaneous death. I put a Blizzard Man in the lava, for instance, and right now he's not even drowning (swimming I guess?), and is in a pool of his own grease (all his fat seems to have melted and drained out of him), full of yellow wounds (burnt and gushing a blank-named material, e.g. "Its fourth finger, right hand is gushing ."), on fire, spewing smoke, and just sort of... there. Basically, it's the Blizzard Man Eternal Flame, never dying, always greasy.
Fire men seem particularly messed up, for what it's worth. They attempt to attack, but when they do, it "goes straight through" (or whatever the message), presumably because they can't attack with their nonsolid form. They also drop "flames" that don't go away, and weird stuff like that. In fact, I'm honestly not sure how combat against those is supposed to work; how do you kill something made of gas/plasma with a sword? At any rate, there are clearly some issues there.
I have no idea if any of this is due to any fundamental problems with the combat system. I sort of hope it's not, and that it's just due to some simple oversights. The new material/body/combat stuff is extremely neat and interesting, so I hope these issues aren't too difficult to hammer into workable condition. Captain also mentioned something about necks/throats not connecting to the bodies/heads properly, so maybe you'll want to ask him about that, although he's probably told you already.
There are some other more mundane bugs, like animals dropping prepared organ meats, bolts not doing much of anything to creatures, and creatures wrestling the teeth out of other creatures, but people have probably mentioned those a lot.