The mechanical properties of glasses from different chemical systems were studied in the light of the atomic packing density (Cg), medium range order and atomic bonding character. The elastic moduli reflect the volume density of energy, and are thus directly correlated to Cg and to the bond strength. Nevertheless, the packing density has actually the greater influence on the final result. In the case of metallic glasses, we found that the electronegativity mismatch (Δe-) between the host- and the major solute – elements provides a plausible explanation to the large variation observed for Poisson’s ratio (ν) among metallic glasses (MGs) notwithstanding a similar Cg. This correlation also holds for monoconstituent oxide glasses and hence provides an explanation to the variation of ν observed for seemingly “isostructural” glasses. In the search for more ductile glasses, the bond directionality and the sensitivity to volume change are key parameters. The energy required for a stable crack propagation through the system, where the crack tends to follow the easy paths, provides some insight into the energy landscape, with a remarkable consistency between the energy as calculated from the bond density or from self-consistent fracture toughness measurements.