Update - Here are some additional details based on the questions and feedback we're seeing:
The new warrior Mastery is 1.5% block and 1.5% critical block per point of mastery.
Shield Block will spill over into critical block if any portion of the 25% would have been wasted because your block + dodge + parry is so high already. If you are somehow always over 100% (which will be pretty hard now), you only get the critical block when Shield Block is up. The intent is to make sure Shield Block isn't wasted, not to make sure you have mastery coming out your ears (or any other orifice in the facial region).
Shield Block is not intended to be a magic damage cooldown (neither is Blood Shield, which also is physical damage only). We don’t want to duplicate the exact same ability on every tank when we can avoid it. Bosses generally don’t spam out a lot of magic damage on short cooldowns. On the other hand, Shield Block has a much higher up-time than Barkskin. We still believe the Cataclysm raiding environment is going to be a lot less centered around tanks dying to spike damage than players currently experience at level 80. Mitigating a lot of damage to save healer mana can be very valuable in Cataclysm.
Savage Defense is massively overpowered on beta and has been dealt with accordingly. Many of the reports of beta raid bosses two-shotting tanks were because that was how much damage the bosses needed to do in order to challenge Feral tanks. With less damage absorbed, the boss damage can be adjusted downwards. We've reduced the multiplier for attack power on the absorption effect (from 65% to 35%, still modified by mastery) and it no longer procs from periodic critical hits.
Paladin tanks are not intended to go cap block as fast as they can. It’s fine if you want to do that, but we don’t treat it as “the new defense cap” and we don’t balance paladins assuming they have a 100% block chance. That is something the community identified as being not only possible, but likely, and one of the reasons we changed the way block works for paladins.
These changes are all slated for patch 4.0.3a when the shattering of Azeroth takes place, and are not currently implemented on live realms. Some of them may be active in the beta test – the Savage Defense change for example was applied via a hotfix to the beta realms recently.