Should DK tank be stacking mastery over stamina or other way round?
The answer is both are equal (but stamina still slightly more important). How so?
Lets compare a situation where deathstrike does 7% of max hp most of the time (which is the case in current tier, it will take the tank to get 70-80k damage in order to break the 7% limit, which is almost half hp gone.)
DK tank with high hp and relatively low mastery VS DK tank with low hp high mastery
High Stamina tank with just 110% shield:
Raid buffed stamina 210k's will give him a 16.1k blood shield
Low stamina tank with 150% shield:
Raid buffed stamina 160k's will give him a 16.8k blood shield
Lets talk about the high stamina tank, he got his trinkets with stamina + all gems with avoidance + stamina or pure stamina gems. With stoneskin gargoyle enchant and pretty good gears with stamina here and there.
Whereas Low stamina tank have trinkets that is fully mastery ratings and all his gems are mastery +40
So, the theory people always accept is tank should never stack stamina like in wotlk. Is it really true?
Not to a large extend, its still viable and infact might be very important for a tank to have alot of stamina. How so again?
We must understand (im not sure about warrior/pally/druid) that a DK tank requires minimal amount of hit/expertise and reforge them to avoidance and mastery as much as we can. In short, sometimes we will be missing some of our Deathstrikes.
So how does this affect the high stamina tank?
It will not be dire for the high stamina tank because he has alot of hp to take damage and healers can top up within 4-5 seconds.
So how does this affect the low stamina tank?
It does alot, if the tank misses 1-2 deathstrikes in a row, it spells death or very hard time for healers throughout the whole raid to top up the tank.
So we can just cap our hit/expertise so we will never miss a deathstrike?
Nope, it is not viable to cap hit/expertise and sacrifice mastery/parry/dodge. If thats the case, you will end up being a low stamina low mastery DK tank which is bad.
So how can you explain why DK does not need that much hit/expertise? I even seen DK who has zero hit and minimal expertise rating and still do well in Heroic raids?
We are not DPS, what do i mean? Ok lets make an example, a frost DK strikes 100 melee strikes in 1minute. If this frost DK does not have hit and expertise, how many of the 100 strikes will he miss? probably 20-30 strikes which is VERY significant and loss of huge dps.
So what about a tank? In a 1minute time frame, how many strikes or how many deathstrikes can a DK strike in 1minute? A good DK tank who time his DS properly will do around 12-15 DS in 1minute roughly. So with that low hit and exp, he might miss 1-2 DS if you count it. As mentioned, we are not dps, our miss is not a DPS concern, its a survivability concern. We all know clearly, a viable tank in current raid must let the boss hit you without heals for at least 5 strikes and still survive. That is a viable tank, if he misses 1-2 DS in a 1 minute time frame it will not kill him.
To top it off, runes are not used if the strike missed, so in short, it is just a GCD wasted. Any dire situation that arises, pop AMS if magical, pop IBF if melee. A well timed DS will put up a 40-50k blood shield along with healing the DK.
Will it affect the DK's ability to hold aggro if he has low hit/expertise?
No to a large extend, from experience, i pull with DND on the boss + dark command. The hunter and rogue will both MD and tricks, according to omen, the 1st 20 seconds of the fight im holding 300-400% aggro, and as the fight goes on, my aggro stays at 150-170% due to vengence. So no worries on aggro unless you totally auto attack.
A side note on parry/dodge importance:
Parry and dodge is different from a warrior, druid or paladin's one. Their avoidance is simply to mitigate or totally avoid an incoming damage and thats all to it (correct me if im wrong here, i dont have those tanks experiences) But for a DK, If you land a DS that is 40k huge, and manage to parry or dodge the next attack, you will possibly be stacking double blood shield, which means if your previous shield is 40k and the next is 15k-20k, they both add up to a 60-70k shield, literally makes you "melee invulnerable"
Therefore Stamina >= mastery > Parry=dodge
Feel free to correct anything i mistook in the post
The above are just from testing and experience, im 7/13 in a small raid guild