Not to mention RIC, ABE, MDT, ACY, SFW, HPN, ISP, AVP, ALB, BDL, HVN, PVD, MHT, PWM, etc.... aside from airspace congestion, think about this: if you consider all the flights in North America, how many of them are to/from these airports? I'd be willing to bet it's 20-30 percent, at a minimum. If several airports get delayed, imagine the ripple effect as those a/c are then not available to run other, non-Northeast megalopolis flights that day.

In ATC terms, there's NOTHING worse than a blizzard or a line of thunderstorms parked over the DCA-BOS corridor, save a 9/11-like event.