Cost really isn't a barrier when you get into certain classes of cars. At the end of the day, safety requirements dictate A LOT about modern car designs, particularly in the US market.
Probably one of the best examples were pedestrian impact requirements in the mid-1970s which required both a raised ride height and "5mph" bumpers. The bumper requirement could basically only be achieved big, blocky rubber bumpers. Even cars designed and put into production at the time-like the Porsche 924 my dad had years ago-were plagued by it. As much as folks love to hate on the rubber bumper MGBs, if you look at their design vs. most everyone else at the time, the folks at Abingdon actually did a halfway decent job of integrating the shape of the rubber into the entire front grille of the car rather than just slapping on rubber blocks like everyone else.
The height regulations for bumpers(again, pedestrian impact) were really detrimental for appearance. The band-aid most cars used at the time was to just raise the suspension which not only looked silly but also ruined the handling. MGs(I know them best, which is why I keep mentioning them) did better sway bars in the later years that brought back some of the handling, but raised rubber bumper cars are nowhere near as nimble at the earlier chrome bumper cars. If you look at modern designs in that vain-the Mazda MX-5 Miata being the best known-they've incorporated the height requirements into the overall design of the car and kept the ride at a reasonable height but the high bumpers still give the cars drastically different designs.
There's also no way around the fact that crumple zones, air bags, and the like both add weight and bulk to a modern car. It's amazing that Mazda has kept the MX-5 at a shade over a ton(I think 2200-2300lbs) without resorting to exotic materials-something that's prevalent in today's super cars. The MGB was one of the first cars built with engineered crumple zones and the folks at Abingdon were proud enough of that to display a crashed one in their factory, but it still pales in comparison to the safety of a modern car(assuming that your MG is actually is still mostly steel and not iron oxide
) I won't complain about the positive impacts of pollution control in terms of cleaner air, but they also dictate a lot of complexity under the hood that takes up space and adds weight. Earlier pollution controls absolutely killed power because of the compromises needed to keep emissions legal. Compression ratios dropped, smaller single carburetors where there had previously been multiple ones became common(and the corresponding intake strangling), and early catalytic converters restricted exhaust. Now, fuel injection combined with electronic engine management and more advanced engine design allow cars to turn out jaw-dropping performance numbers(your average family sedan can now keep up with your average 1960s muscle car) while also pumping out amazingly clean exhaust and impressive gas mileage figures. Still, though, all this extra stuff does change the look of a car.
One thing that has changed in the last 30 years is headlight design. In the US, up until(I think) the early 1990s all cars were required to use a sealed beam headlamp. There were basically two kinds-the round 7" lamp and the rectangular one. Dropping this requirement has had probably the single biggest impact on the design of vehicle fronts and has also-in general-allowed more efficient reflector design to improve visibility without adding glare for other drivers. It has also, unfortunately, resulted in some absolutely ugly headlights in the name of aerodynamics.
