My initial assessement on the priest is that Holy appeared initially to be for Tanking mainly since its Renew and generally single target healing is huge, however, since it also has a "mode" for AOE, I think what we have is:
1. An "arcane", novel spec, Discipline, which can do both but it's a bit weird. I mean, it does healing in conservation while DPSing, how weird is that? [plus it heals in AOE by popping some automatic shields and other weird things like that]
2. A traditional spec. It will do big healing on tanking and big healing on AOE, but, it needs some traditional "skills" [the switching [mechanism] from single target to aoe healing is technically 'weird' but practically once it's done healing returns to 'tradition']. e.g. It won't save you easily with popping a shield for a while. It will need your proper big heals with a rotation-ish.
What do you think?