Considering all the tentacles and claws and whatnot are actually parts of Deathwing, I have a hard time considering MoD to be an "add fight." We're literally interrupting his Armageddon spell by hacking off his limbs until, like the Black Knight, he's just a torso shouting imprecations. If you look beyond the immediate area while fighting him, you can see him waving his stumps around and gnashing his teeth. Even in phase 2, when he hits the ground, those tentacles aren't distinct entities - his head puts them down like roots to attack the raid.
Granted, it does seem like we're not having much effect on him. That's the fight's only problem, really. Deathwing is just too damn big. He's visible from space. A raid attacking him is like ants attacking a kid with a can of gasoline. This isn't even a problem that fight mechanics could address. He's that big in lore. And having him drop to human form is ridiculous, since dragons only go human form when they want to be sneaky or not alarm the lesser races. Deathwing doesn't do sneaky. Alarming people seems to be his hobby.
This fight is the closest we've yet come to fighting an Old God which is completely unchained. The difference in power between him and us is incomprehensible. Blizzard went and made him too epic. He reached the pinnacle of the epic bell curve and went straight down the other side, and the kicker is that there is nothing that they could have done about it.