There are a number of stat and mechanic changes being announced for 4.0. A full list is available here, but these caught my eye
Block Rating - Block is being redesigned to scale better. Blocked attacks will simply hit for 30% less damage. Block rating will improve your chance to block, though overall block chances will be lower than they are today.
I wonder if the developers will leave critical block as it is (ie 60% blocks). If so, and using current encounters for illustration, we might expect blocks of 15k to 20k in 25 man content. That's quite a lot. I know that the intention is that bosses won't hit so hard in Cataclysm, so these numbers don't quite correspond to what we'll see in 4.0. Nevertheless, block will remain a very powerful mitigation/avoidance tool, and it's still not going to be easy for Blizzard to balance blocking classes (particularly warriors) with non-blocking classes.
With lower boss damage per swing and mana management becoming important again (this is the stated design goal anyway) avoidance may be as important as stamina and mitigation in Cataclysm. Nevertheless, since warriors can block it makes sense that their other avoidance and/or EH should be lower than druids and DKs to compensate for this. It will be interesting to see if we get lower EH (as we have now), and whether this actually matters.
Parry - Parry no longer provides 100% avoidance and no longer speeds up attacks. Instead, when you parry an attack, it and the next attack will each hit for 50% damage (assuming they hit at all). In other words, Dodge is a chance to avoid 100% of the damage from one attack, Parry is a chance to avoid 50% of the damage from two attacks, and Block is a chance to avoid 30% of the damage from one attack.
Two points - firstly on the basis that spiky damage = bad, this will make parry better than dodge [Edit - no it won't, because you can block, dodge or parry the second attack, or it can miss. This means that parry will have around 75% of the value of dodge, depending on your avoidance levels] Good for DKs, bad for druids. Secondly, I wonder if this mechanic will be applied to mobs and bosses as well. If so, the value of expertise plumments for tanks, because one of the main reasons we take it is to avoid being parry-hasted. There is no mention of expertise being removed in the blue post, so I guess it just becomes a dps stat.
EDIT: Been thinking about it. If you can avoid the second attack then this is a massive nerf to parry because it is unquestionably worse for mitigation purposes than dodge, even before DRs kick in. But if they make it so you can't avoid the second attack, who would ever stack parry because parrying an attack means you can't dodge the next one - it lowers the value of your dodge stat! This change can't make it to live surely?