Serious changes to healing throughput as percentage of total health in Cataclysm

We knew this already, but the Paladin preview that came out today had an interesting tidbit:

Furthermore, the healing environment of Cataclysm is going to be different such that a paladin may not be able to fully heal themselves during the duration of Divine Shield to begin with, so this may not be a problem. 

Divine Shield currently lasts 12 seconds.

Blizzard is contemplating having a Paladin, currently the highest throughput single-target healers, be unable to heal themselves to full over a 12 second time span.  That is a large, large change.  That may be even slower than healing in vanilla was, and it suggests that tanks will either have enormous health or will take very low damage relative to their total health pool.