Truthfully, my biggest fear would be the Holland and Lincoln Tunnels...
The loss of life wouldn't be that large a number, but the problems this would cause the city would be almost unmeasurable... There is a huge security presence in the area at all times, which is somewhat comforting.
Agreed re. the loss of life...I can't even begin to think of the impacts though. I used to take NJT into the city every day. It was crowded and a pain as it was. If the tunnels were out, trains would become unbearable. If I had to do that, I'd probably buy a kayak and park in Jersey City or something. Not even kidding. So glad I don't have to do that commute anymore.
re. Security there...I see lots of cops, but it's not like they actually stop anyone. I know they make random box trucks stop, etc, but I've often wondered how hard it would be to load up a normal minivan with explosive and get it through there. I just don't feel that it's THAT secure.