Has anyone actually done the mathematical comparison between elixirs and flasks in terms of EH? Assuming our gearing is flexible enough that we can use the mastery elixir to hit block cap, how does 900 armor + 225 mastery stack up against 300 stam in terms of EH?
Comparing Health vs mitigation is dicey thing right now, because it comes down to a number of variables that aren't entirely clear right now, such as how bursty the damage is, ratio of blockable to unblockable (incl magic) damage, what form the "tank threatening" bursts of damage take, etc. For example, Mastery and Armor are worthless against magic damage, so if you're OT on some Hydross-style fight, then the STAM flask is going to win outright (though a resistance flask might be an alternative).
Another minor point (thats irrelevant to everyone but Alchemists) is that its not clear whether the flask Mixology bonus % applies to elixirs the same way it does to Flasks.
That having been said, from an itemization points standpoint, the value of the combined elixirs kicks the crap out of the flask (part of the problem is that the flask is significantly under budget). Also, from the data we've seen on the Cata raids from beta (which may or may not represent their final tuning level), the elixirs are likely to be more valuable across the fights we've seen so far, and if Alchemists get the Mixology flask bonus equally applied to Elixirs, Alchemy will probably be a min/maxing choice for tanking professions.
It depends, but probably not. First, at 85 it takes a good deal of +hit to get capped. Second, Theck has shown that at just about any likely gear-level, Mastery is going to be superior to any avoidance stat for damage reduction, which is purely a side-bonus to the actual goal of reaching block-capped and thus dramatically smoothing your stream of incoming dps. Also factoring that survivability is expected to be a far larger issue than threat, you should reforge your highest point stat on each piece of gear to Mastery. At the moment, the biggest question (and which I expect to spawn significant debate once people get some real data on the hard modes) is whether to stack Mastery (assuming below the cap) or Stamina. Stamina seems like the winner at the moment, but if tanks aren't really in danger of being 2-3 shot even in hardmodes (plus, see the notes above to Capstone), then Mastery will be very competitive.