Wow, it’s weird the entire mastery discussion is totally revolving around one specific ability. Well…maybe not that weird…heh.
Anyway, the two arguments against DoaM seem to boil down to two camps:
1) it’s too powerful/legendary/MJ-esque to give to a lowly 1st level fighter, or
2) it makes no sense logically (a miss is a miss).
From where I sit, the counters are:
Too Powerful: obviously I can’t know for sure, but I’d wager at most we’re talking a few points (maybe even only 1). And only for a very small handful of weapons. And maybe it’s me, but I’d rather take the ability to gain Advantage, or off-hand strike for free over a guaranteed 1pt of damage a round. No one is going to be sitting around saying, “Thank the gods you could DoaM! Otherwise that Tarrasque would have been a problem!”.
A Miss is a Miss: lots of people have already described how this makes narrative sense descriptively. And HP have never been Body Points. If they were, what is the logic of a character suddenly being 5 times tougher than they were six months earlier? Is their skin tougher? Are they suddenly as big as an ogre? Man, that’s a lot of Wheaties they’ve been eating! And as for the poison issue, it’s dead simple to add a clause that says “special effects (such as poison) don’t trigger on DoaM”. And/or "DoaM can never reduce a creature below 1 hp".
Without any DoaM weapons, it’s theoretically possible for someone to go “I can do this all day”, if they are lucky enough in a fight. All DoaM does is say that there are a couple of weapons that are brutal enough, that when wielded by a combat specialist trained in their use, that you’re eventually going to get worn out and beaten down, so better finish it soon.
I’ve said before, I’m good either way they decide to go. I would like to see some actual specifics before wasting too much energy arguing the theoretical concept. And I’d love to hear what people think about the other Mastery abilities.