  We revisit the concept of marginal stability in glasses, and determine its
range of applicability in the context of avalanche-type response to slow
external driving. We argue that there is an intimate connection between a
pseudo-gap in the distribution of local fields and crackling in systems with
long-range interactions. We show how the principle of marginal stability offers
a unifying perspective on the phenomenology of systems as diverse as spin and
electron glasses, hard spheres, pinned elastic interfaces and the plasticity of
soft amorphous solids.
