In this thread, people have talked about restrictors and aero and powertrains (especially clever diffs), and how they affect "spectacularness".

But I wonder, is suspension a bigger factor? Group B cars had a lot of power, but they also bounced around on uneven ground. It seems to me (not an expert) that modern suspension is so good it just eats the bumps and makes a lot of rally stages look not so different from circuit racing as the car goes between corners.