As far as AMZ goes remember that it scales from strength not attack power so it is much stronger from a dps DK than from a tank one.
Not *that* much stronger in a raid context, because the scaling is basically irrelevant. Either a tank or a DPS value gives it enough power that it will absorb roughly 75% of the first "tick" of damage for every person standing in it, but not enough absorb to last past that.
And technically it's only so good because the devs never get around to fixing it. It's not supposed to absorb a big burst for everyone inside it, that's only because the code isn't fixed (which I assume is the dev way of keeping it from being even worse than it is).
Sure, but "technically" or not, that's how it functions in-game. I just think it's important to be honest about how good it is - it's definitely weaker than most other raid cooldowns, but it's still significant enough to be worthwhile against magic damage and the 2min cooldown time is a distinct advantage in some situations. It could probably use a flat out redesign so it works like you'd expect it to (reading the description, you certainly come away thinking it's total crap).
To be honest, thinking about it, I'm not even sure what purpose the "cap" serves. In basically any PvE situation it'll absorb 75% of the next attack against everyone in the radius, and then go away. Why not just make it absorb 75% magic damage for 3s and call it a day? That preserves its unique "flavor" (strong against single hits, weaker against AOE ticks), likely buffs it in a number of situations where it's currently a bit weak (basically the more frequently AOE dmg ticks, the weaker it is) and probably won't make it overpowered in any situation where it wasn't already going to be overpowered in its current incarnation.
I agree that the radius is small, and with the radius buffs to Swiftmend/etc incoming it'd be nice to see them buff it up for AMZ too. It requires more precise stacking than basically any other ability in the game.
I'm not convinced that having it as a talent is really all that problematic. Paladins have a tier where they pick between personal survivability (Unbreakable Spirit) and raid utility (Hand of Purity/Clemency) too.
I think the "problem" with that talent tier is that Purgatory shores up an obvious DK weakness (the fact that they tend to be more spikey than other tanks). I'd prefer to just see them address the weakness directly by making DKs slightly less spikey, and leave Purgatory vs. AMZ as one of those interesting choices to make depending on the encounter.
I mean, on paper, Unbreakable Spirit is an amazing personal survivability talent - Divine Protection at 40s instead of 60s is a massive improvement. In practice, it turns out that Hand of Purity and Clemency get picked for a lot of fights, because they are equally big raid survivability/utility boosts. I suspect that if DKs weren't livin' life on the edge so frequently, Purgatory would feel a lot less mandatory.