Gauge theories arising from the compactification of string / F / M-theory are strongly constrained due to consistency conditions on topological data; for example, rk(G) can be bounded and the representation theory is limited. I will discuss constraints on chiral matter spectra which are necessary for string consistency but go beyond those necessary for the cancellation of gauge anomalies. The differences between string constraints and QFT constraints can be understood in terms of transitions in the landscape. I will argue that they provide a useful, perhaps new, guiding principle for physics beyond the standard model.
As a rich example, I will present a class of models where quasi-hidden sectors are added to fix otherwise inconsistent MSSM quivers. We have dubbed these models "Stringy Hidden Valleys," and happily they exhibit a number of generic features. These features include new electroweak multiplets, an anomalous U(1) symmetry which protects messenger masses and has strong consequences for superpotential couplings, a rich phenomenology of heavy and light Z' bosons, and axionic couplings required for anomaly cancellation. I will discuss possibilities for dark matter and supersymmetry breaking in light of these generic features.