FWIW, I think this impacts only the relationship between federal agencies. I haven't found a single reference that describes how this designation impacts the relationship between FEMA and the state, and lots of references as to how this designation cuts inter-federal red tape to the benefit of FEMA itself.
Anyway, it's up to the government players to figure out how they will cooperate, and it's up to the voters to say, "Enough already, just give up on it, stop overpromising. Let us take care of ourselves."
And yes, there are a number of us dorks out here ;-)
It strikes me as a more or less meaningless bureaucratic designation. There are many vague references about coordinating and communicating, but when you boil it all down, designating this an "Incident of National Significance" amounts to stepping in front of state and local resources. It seems pretty clear from reading the NRP that it is really meant to cover WMD attacks, in which there may not be too much in the way of local authorities, or at least not any with the means to deal with such incidents. Although natural disasters are certainly mentioned in the plan, they seem to me to be almost an afterthought, and there are repeated statements that things should always be dealt with at the lowest level of government first.
I can't find anything in the NRP that really changes, fundamentally, the way the Feds respond to hurricanes. That may well change now - but this is the first time the designation has been used.