I think Blizz has made an effort to make the tanking off specs (ret pally, fury and MS war) better at tanking through allowing threat generating skills to operate in the correct stance/aura and also allowing pally spells to scale with atk pwr, so this is not such a problem. It definitely will mean less mitigation however so in general you will have more room for error with a prot specced tank, but off specs will definitely be more capable than previously.

My preference will be for the dual spec system to allow me to use my pally in ret form for PvP and prot for PvE. Ret pally leading 4 shammies around the block will make for some serious demolition, even if the shammies are all specced resto and just spam heals on the pally