- Block will be heavily redesigned. There will be no more block value of any kind and all blocks will remove 30% of a blocked attack's damage. Block rating will still improve your chance to block as it does now, but if you do block an attack, rather than the current system where you block (as an example) 2000 damage because you have a shield block value of 2000, you'll block 30% of whatever the damage would have been. This means a gimmick block set won't make you invulnerable to trash mobs, but also that block will be more useful against bosses who hit for 50k. (A boss who hits for 50k now would hit for 48k using our hypothetical block value. He'd hit for 35k with the new system, as 30% of 50k is 15k.) This also means that Shield Slam will need some other method of calculating its damage.
- Parry is also changing. Right now, parry basically works like dodge: a parried attack does no damage. In the new system, when you parry an incoming attack, it and the next attack will hit for 50% damage. This takes parry out of the 'all or nothing' club of avoidance abilities (and leaves dodge alone as the sole pure avoidance stat) and makes it a triggered mitigation ability like block. Parry hasting will also be removed. (What effect this will have on expertise is as yet unknown.) Whether or not your parry damage reduction will be used up on a dodged attack, I can't say at this point.
- Defense is gone. We knew this, but there it is. Combined with the changes to block and parry, it's safe to say you'll be getting hit more often for less damage (since defense also provides a chance to not be hit at all in addition to its chance to not be critically hit). At present the plan is that you will become immune to critical hits just by switching into defensive stance. Since we know that resilience is changing to only work against player damage and not crits, mana drains, etc I would expect the crit immunity provided by defensive stance will not work against players, but that has not been stated yet.
- Mastery will be added. Mastery is still in the 'it will be really cool, but we're not ready to really tell you what it does' phase, but it's stated to be directly tied to your talents and talent choices so that having more mastery on gear will allow you to do whatever it is you do (tanking, in this case) better. Perhaps by increasing how much health Last Stand gives to pull an example out of my butt.
- Stamina on non-plate and non-tanking items will be going up, and tanks can expect that their gem item budget will be taken out of the stamina on an item, as gem budget is usually taken out of a piece's most attractive stat. Health pools for tanks will be lower, proportionately, when compared with non-tanks.
- They've specifically mentioned bonus armor as something that will be coming down. In addition, they're reducing how effective plate will be in mitigating damage compared to other armor types.
So these are basically the changes we can expect. Raw stats will still be on gear, and stats like expertise, hit, dodge, parry and block will still exist (if heavily modified in some cases) but defense will be gone and warrior tanks will have lower proportionate armor and stamina compared to non tanks. It basically looks like the design is shifting away from huge bursts of damage (the 'non-crushing crushing blows' of Wrath) which made designing Icecrown require Chill of the Throne to balance out, and towards more mitigation.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.