JBV suggested I turn some comments into a question, so here goes.
Another question  asks about applications of QM computing. One answer  was "efficiently simulating quantum mechanics". Apparently this idea dates all the way back to Feynman's early writing on subject; although I dont have a reference. So:
Question. What is the proof that a quantum computer can efficiently simulate an arbitrary quantum mechanical system?
On one level this seems basic. However, this does not seem to be trivial for following reason: most quantum computing literature seems to reduce to operations on gates acting on two particles or other small subsystems. (Yes, Toffoli gates act on 3 inputs, but anyway are often reduced to two-qubit CNOT gates.)
There is surely no question, due to Turing completeness, that a quantum computer can simulate arbitrary classical or even quantum physics (although maybe there are some naysayers there due to the uncertainty principle etcetera — I would be curious to hear about that too). But it seems to me that to simulate arbitary quantum physics efficiently one at least needs a way to simulate arbitrary n-way interactions in mostly/nearly 2-way gates.
One could argue that we can build arbitary n-way gates, but the clear evidence after many years of experimental research is that even just 2-way gates are extremely hard to build, and that n-way gates would surely be much harder. (There are some 3-way quantum experiments, e.g. 3 particle bell inequalities, but they are difficult to build.)