They were fast compared to the average car back then. Basic econoboxes today run fast compared to the old stuff. On the other hand, when you drive a vintage muscle car, you DRIVE it. Newer cars, including the hotter ones like the Mustang you mentioned, aren't really a drivers car. You are just along for the ride actually. Cars today are too easy to drive.
Porsche at least remembered that their customers actually LIKE to drive. That's why Porsche, which is not know for being a cheap car, still offers a manual transmission. Try getting one in a domestic car or truck today? None of the big three offer a manual transmission in their 1/2 ton trucks. Period.
I see a few manual trnsmission starting to creep back into American cars (Cadillac and Buick both offer them, I think). When you get down to it, the old muscle cars had souls. The new ones, well, I'm sure they're nice cars but they really don't have much character.