Quantum computers don't really compute many things in parallel. Or rather, they do, but the problem is getting the results out.
To stick with your ray tracing question, you could probably indeed trace an exponentially (in the size of the computer) number of rays, but at the end you have to do go back to the classical world to actually read out data, and you'd essentially be able to only read out the result of one ray picked at random.
Quantum computers are only know to provide exponential speedups in cases where there is some algebraic structure to exploit when condensing the exponentially many computations down to a single result. That's why you can get an exponential speedup for factoring or discrete logarithm, but not for symmetric cryptography or really any other useful problem (except for the kind of circular problem of simulating quantum systems).
That's what makes me personally hope that quantum computers will never scale. The likely outcome of quantum computing is the worst of all worlds: all known practically useful cryptography will be broken, and practically nothing will really benefit.