Well, for starters, I think mastery is going to go up in value immediately - I don't expect we'll see readiness or multistrike appear on our gear from SoO or earlier in Mists, so I expect the hit and expertise will be replaced by crit and haste. With warriors getting a clone of the paladin Sanctity of Battle ability, almost all tanks will at least be able to make use of haste (it's still not going to be great for prot warriors, but it won't be useless) so the crit and haste I expect to replace hit and expertise will be of value. But with mastery adding a starting 12% to Attack Power, and scaling upwards with mastery on gear, for the month or so we're still in Mists content mastery is going to be something we really want because it'll be feasible to push that mastery attack power up to as much as 50% with end of expansion gear. (Whether or not this actually happens depends on how it's formulated, of course.) Right now every tank gears differently - some like haste and mastery, others despise one or both. But I expect mastery to be the winner for at least a month.
Once we start gearing in Draenor, all bets are off, of course. We know from datamining that there are efforts to make the new stats add value for tanks such as this Blood Craze ability for protection warriors, and we know what Readiness is for each tank (again, an example or two may be helpful) but that just gives us an idea of what these stats will do - it is as always subject to change. I expect Readiness to be more attractive than multistrike, however. It'll depend on class, but the wide array of abilities affected by it compares favorably to the trinkets from Siege of Orgrimmar, and from experience I know I like having Shield Wall on a 1.5 minute cooldown. Having it affect a variety of abilities like this just makes it even more attractive. Of course, it depends on how good it is per tier - if it barely has a noticeable effect in the first tier of raiding, it's value will diminish, but if it's too good too early it'll end up getting nerfed later.
Despite the Vengeance to Resolve change, I don't think tank DPS is going to plummet - unlike right now, where some tanks wear basically the same gear as DPS and others use some defensive stats, all tanks are going to use the exact same gear as DPS. Considering the change to mastery will be adding scaling attack power (beginning at 12% and going up from there) our DPS should go up reasonably well as our gear does. Plus, all are going to have the same amounts of strength or agility as DPS (mostly the case now, anyway) and we'll be using the same offensive stats. It comes down to how abilities are tuned - if the target of 75% of a dedicated DPS player is hit, everything should be fine.
I know some tanks are going to miss pushing 400k (which the squish is going to end anyway) and seeing themselves on top of the meters, but it's just what's going to happen. As such, with tanks not putting out that kind of damage anymore, there may be less of the 'we can't bring X tank, that class doesn't do as much damage' that we saw in Mists of Pandaria. I'm of the opinion that Blizzard is making the change to Vengeance less for the good mechanical sense of getting rid of the difficulties it causes with things like tank swaps and more for the imbalanced damage it caused, which was always a nightmare to balance and which made things like Riposte necessary.
I do wonder how the Draenor Perks are going to affect play. Some of them are pretty basic - brewmasters and guardians get 20% base stamina, prot pallies 10% damage reduction to magic damage, warriors 15% armor, while death knights get a rune shield from their rune strikes that absorbs extra damage - while others make abilities hit harder or remove penalties or down sides to those abilities. I worry a bit about giving some tanks stamina - it's the old problem we had back in Burning Crusade where some tanks were seen as mana sponges - but we'll see how it plays out. A lot of the perks are just flat damage boosts, which is the case for DPS perks as well - frankly I think those might need to be buffed to keep up.
We're still waiting to get to see how this all shakes out. My perennial concern is for rage tanks, as it always is - starting at zero rage puts them at a disadvantage, since they can't have their active mitigation as accessible to them since it costs a resource they don't have to start with. Hopefully bears and warriors will generate rage fast enough that it won't be a major concern. Otherwise, tanking in Warlords of Draenor appears to be a continuation of what we've seen in Mists, rather than a departure.