What are the chief hurdles in unifying all forces, gravity included, that particle physics faces today?
Does there have to be a quantum theory of gravity? Not unless it makes some testable predictions. The Large Hadron Colider (LHC) experiments [at CERN] may tell us something about gravity, if the extra dimensions notion [as required by string theory] turns out to be right. I am more focussed on more modest questions, such as an explanation for the curious spectrum of quark and lepton masses. Your remark – “does there have to be a quantum theory of gravity?” – is somewhat surprising because quantum theory tells us that if there is a force field there must be a quantised manifestationof the same. Isn’t that so? Probably yes… but the quantum aspects of gravity are likely to be unobservable. We have not yet observed gravitational waves… although we may do so in the near future, There is simply no way to observe the particle aspects of gravitons [the quantum of gravity analogous to the photon of electromagnetism], such as the gravito-electric effect, because of gravity’s weakness. Of cou
