Blocking via a shield has always ben a problematic mechanic and Cataclysm really just made it worse. It went from irrelevance outside dungeon runs to near omnipotence. And truly, there isn't an easy fix.

My suggestion is simple....drop the blocking mechanic from Paladin tanks. Or, more specifically, keep it for Holy and Ret style tanking allowing them to use SotR for the block chance increase while Prots revert to a more appropriate style of tanking. I could suggest reverting DK tanks to Dual wielding and allow paladins to assume the 2H tanking role, or have Paladin tanking based around shield or heal mechanics.

I also think the new active mitigation system, fromt he litle info Blizzard has provided so far, is likely to be a damp squib and simply introducer complexity for the sake of complexity. Paladins tanks are supposed to choose between increased block, a heal or a shield but its hard to credit that setup as anything special....

Then there are the tanking stats. Parry and dodge are effectively the same system with different names, while hit and Expertise are linked in concpet but both are fairly cap-able.

Ideally, IMO, Parry and Dodge would be separated in some fashion - either that or combined. The idea of giving tanks a choice of two between Block, Parry and Dodge is also there but strikes me as inherently limiting in its own way.

The Mastery could then be used to fill in for or improve the third mitigation stat...warriors get block, paladins, druids, dks, monks would use shields, heals, avoidance, or whatever.