There were very few cosmetic changes. The majority of the changes involved safety and emissions related items, most of which would become effective on 1/1/72. A couple of safety-related items was the passenger side mounted seat belt light that actually went into production on 12/1/71 on the Mustang line. The Mach1 pop open fuel cap, which was prone to leakage from rear-end collisions or rollovers, was replaced with the twist on the standard fuel cap, although some early production '72s still had the pop open cap.
There was also a difference in how horsepower was now measured. The previous method was Brake horsepower which was measured at the flywheel with no water pump, alternator, PS pump, etc. The new SAE measured power at the flywheel with all the normal power-robbing accessories in place.
For 1972, Ford had cut compression drastically and cam timing retarded 4 degrees on the majority of all gas engines in the entire car a light truck line. With the looming gas crisis/oil embargo on the horizon, Ford knew that the ability for all these engines to be able to run on the cheaper regular fuel would be a great selling point.