This is a classic "Nor'easter" the likes of which we don't see more than a couple of times every few decades. The biggest threat, as others have pointed out, is the tidal surge from the storm itself -- exacerbated by the wind out of the east driving waves into inlets and river mouths along the East Coast. This means there will be a surge of ocean water up into these areas while 5-10 inches of rain is falling in the interior of New Jersey, New York and Pennsylvania. Quite simply, the runoff from the rainfall will not be able to drain out of the watersheds as quickly as usual because of the higher water levels downstream.
Some folks are comparing this to the infamous March 1993 "super-storm," but my recollection is that it may be more similar to the December 1992 storm. For those Freepers who remember that one, it occurred just before Christmas in 1992, and the storm surge was unlike anything I've ever seen in the NYC metro area. Not only did the storm peak during a full moon that made the tides that much higher, but there was also a total lunar eclipse during the full moon that month -- which meant the storm surge came on top of what was an extraordinarily high tide.
Understand the full moon's relationship to a high tide. But what would a lunar eclipse have to do with it? A shadow has no mass...