In terms of cosmetics, the worst thing to happen was the NHSTA mandating minimum height requirements for front ends. This has caused everything from the grille to the wheel wells to get consideredy bigger.
Interesting, I did not know. With a search I found this:
NHTSAs regulations at 49 CFR 523.5 provide two basic ways in which a vehicle can be classified as a light truck for CAFE purposes: 523.5(a) covers vehicles that the agency considers functional light trucks, that are not passenger cars because they were not manufactured primarily for transporting up to ten individuals; and 523.5(b) covers vehicles which are expressly excluded from the passenger car category due to their capability for off-highway operation.
[1] Your question focuses on 523.5(b), which states that a vehicle must either:
(1)(i) [Have] 4-wheel drive; or
(ii) [Be] rated at more than 6,000 pounds gross vehicle weight; and
(2) [Have] at least four of the following characteristics calculated when the automobile is at curb weight, on a level surface, with the front wheels parallel to the automobile's longitudinal centerline, and the tires inflated to the manufacturer's recommended pressure
(i) Approach angle of not less than 28 degrees.
(ii) Breakover angle of not less than 14 degrees.
(iii) Departure angle of not less than 20 degrees.
(iv) Running clearance of not less than 20 centimeters.
(v) Front and rear axle clearances of not less than 18 centimeters each.
Running clearance is defined in 49 CFR 523.2 as the distance from the surface on which an automobile is standing to the lowest point on the automobile, excluding unsprung weight.