This blog is mostly for the lovely Demeternoth to better explain/show how the stats on items will affect tanking. (Be sure to check out her blog as well and she should have some more detailed information coming from the Beta soon!)
Basically I noticed after doing a couple quests (and already replacing my heroic and and non-heroic tier gear with greens) that there has been an overall change in item stats. Overall I think the change can be good albeit being extremely annoying on another level.
Before I go on I'd like to mention a small detail
you'll be able to view in this screen (don't ask about the gem on my weapon, I obviously forgot to change it to a dps one after I went dual wield tank spec): Blizzard has now included a sort of "ratingbuster" (one of my favorite addons) tooltip to the quest reward selection UI. I honestly wish this was present all the time like with rating buster to compare gear in your inventory to gear you're wearing, but for the moment it is only found in the quest reward selection area. It's handy and accurate but just needs to be more available.
Now look back up at the stats on the item.. something you don't see in this screen shot but I can tell you from other plate rewards I have acquired: DEFENSE IS NO MORE! Scared yet? Well it's not as really bad as it sounds...
Consider druids. (In my opinion the most OP class in game due to their versatility).. you can pretty much tank in any gear as a druid because your crit immunity comes from talents. While it may not be the best action, it's still possible. Honestly as a plate tank I have always carried 2 sets of gear in my bags.. my tank gear (which I'm usually wearing as that is my main spec) and my DPS gear.
It appears that stats on most the plate is: Stamina, Strength, Hit Rating, and Parry. Considering that defense no longer exists this essentially means that my DPS and Tank gear could possibly be the same. I'm not sure how this will work in regards to stamina (and I haven't advanced far enough yet in the Beta to judge for certain) as I would imagine that a DPS would not have nearly as much stamina as a tank but... that is where I come back to the lack of a defense stat...
Your crit immunity will be determined by talents. I assume this is also why Blizzard (mainly in regard to death knights) moved all the tanking talents into one tree. If you don't have the talents for tanking, it's not something you're going to be able to do efficiently, simply put.
Again, I can only really go into detail in regards to Death Knights as that is my class, but here is a look at where this will come from.
You'll also notice at the top of the individual talent trees there is a summary of your talents. Here you can see "Damage Reduction", "Vengeance" (not quite sure what this one is yet) and "Life Shield". This summary evolves as you choose your talents and is a good way to see how your stats will be affected since it updates during talent selection, especially if you have the "preview talent changes" feature enabled.
This is where I get back to the stamina question. There is also a talent "Veteran of the Third War" deep in the Blood Tree (under the talent that has the same icon as Empower Rune Weapon) that increases your "Total Stamina by 9% and your expertise by 6." when the talent is full (goes to 3/3). This leads me to believe that there will no longer be separate tanking and DPS gear. Which brings me to:
THE POSSIBLE ANNOYANCE FOR TANKS
Remember how +healing and +damage went by the wayside for +spellpower? Yeah. That lead to healers and DPS potentially wanting the same gear and you can expect that this will be the new issue with Tanks and DPS.
Mind you I always experienced this a bit when I was a 2 handed Death Knight tank simply because the DPS would want the weapon for the DPS and I'd be in it for the stamina. It was never pretty when I would roll on the item, and you can expect this to be the new norm.
I know it will irk me because there's always been that unspoken rule that healers and tanks should have more priority on required gear and I will be a tank fighting for gear to TANK with DPS, but I guess to be honest it's only "fair". (I'm trying to convince myself here..) It takes the balance of the "trinity" to complete any dungeon or raid and therefore each member/role is just as important as the other.
If that doesn't convince you, consider our time has come. Healers have been putting up with it for a while now and druids have been jerking with the system for ages.