Avoidance can be great but on the fights that actually matter at the moment you need to make sure you can take huge hits/strings. Thorim and Iron Counciil hard modes spring to mind as the main culprits. Being able to avoid 3 of Thorims attacks in a row is great but nto at the expense of getting 2 shotted the first time you fail to dodge. These fights you need to stack stamina to be able to take the biggest hit or chain they can dish out. Being able to take 3 hits instead of 2 is far superior in these instances than having more dodge as any bad luck streak will see you wipe. One of the main reasons we're taken to these fights is the huge HP pools we can get too if geared for it. Steelbreaker especially can hit for 60k+ in the final stage and an avoidance tank is unable to tank this with 43k hp or whatever the figure was above.

For other fights like whichever it was above you were on about the same principle applies. You need to be able to survive bad luck streaks of hits. But normal mode bosses in the most part don't have the ability to two or three shot you. Once you reach the point you can safely survive these streaks, then avoidance becomes a better stat to stack from then on. We are far from that point on hard mode bosses at the moment so I suspect stamina stacking will remain dominant at least into 3.2. If healing is changed again to be less effective or healer mana conservation becomes more important then this might change.

I find any discussion of some general formula or rule of thumb regarding stamina vs. agility to be pretty pointless. There are at least 3 major variables that will determine what the proper balance is:

1. What are the boss' specific mechanics? How hard does hit hit with his auto-attack swings, how quickly does he swing, does he have any special abilities that the tank can't avoid?

2. What are your healers like? Do they have the skill and latency to cancel heals to save mana when you're on a dodge streak or do they continue spam healing you?

3. What are your guild's other options at tank? There's a big difference between being the go-to tank for the majority of an instance and the off tank who is only called upon for gimmick fights.

Something that I think isn't pointed out enough is how extra stamina can also be "streaky". A lot of people will say something like "The boss hits me for 22k, so if I get 44001 hp then I'm safe from being 2-shot and can stack avoidance to reduce the chances of taking 3 consecutive hits, which will kill me without heals." In reality, bosses hit for a range of damage and you aren't always topped off. You then fall into the trap of "I have 44k hp and the boss hits me for 18k. I can't get to 54k to safely live through 3 swings, so I'll stack avoidance", when in reality the boss hits you for 15k-18k. While stacking stamina for another 5k hp won't help you live through a worst case scenario, it will help you in the case where you take 2 consecutive minimum swings because you can't be killed by a 3rd swing even if you get no heals. This works similarly for situations where you aren't healed to 100% before an unlucky string of undodged hits.

The fact that stacking avoidance vs survivability is dependent on the fight seems to be universally accepted, and has always been that way. The interesting thing pointed out by the OP is how stacking stats is going to affect us in 3.2 and beyond. Will we be seeings bosses that hit every second for 30k, or are there other directions we'll see Blizzard going with this?

I believe Blizzard has somewhat shot themselves in the foot with Hard Modes. Think of the BC to WoTLK transition. Sunwell gear lasted well into Naxx, and because of this items were forced to progress overly fast. Hard Modes are doing the same thing, really. If you're sitting at 55% dodge or 55k HP right now, you're going to need upgrades. Likewise, if bosses are doing 25k a hit now, they would need to do even more in 3.2.

Unless every encounter is a gimmick fight, there will need to be some form of progression, otherwise the content will be trivial and/or not worth running. And as of now, the only way to progress is with harder hitting bosses, and more avoidance/stamina in an already inflated tanking design.

The Sta vs Agi argument isn't really changed much with this change. It's still an apples vs oranges comparison, except that you need so many apples before you can focus on oranges.

You will still want to stack stamina until you are no longer in danger of dying too quick for heals on an encounter, and then stacking agility after that to maximize mitigation/avoidance.

As a side note, these changes will probably push defense higher than dodge rating. The current Rawr shows dodge at a relative stat value of 53.98 for me and defense at 50.75.

The biggest drawback of this change will be seen by druid main tanks in guilds that are still progressing in Ulduar. If your guild is going to finish in Ulduar, then you will have to deal with the fact that the content was balanced around the old agi/dodge values. This could be offset if the amount of dodge at 0 agi was increased enough though.