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Hund's rule coupling plays a surprisingly crucial influence on the metallic properties of correlated materials. Three main effects of this coupling are isolated in the multi-orbital Hubbard model: it tunes the proximity to the Mott metal-insulator transition, it renormalizes the Fermi-liquid coherence temperature and it acts as a "band-decoupler". These effects can cooperate or compete to enhance or suppress the metallic properties of materials for which this model captures the low-energy physics. A simple rule of thumb to know if metallic, bad-metallic or Mott insulating behaviours are promoted is found, depending on the filling of the system. Ab-initio LDA+DMFT calculations allow to compare these predictions with actual materials (mainly 3d and 4d transition metal oxides) and check that this concise classification allows to capture some important material trends, and explains why very similar materials (such as SrVO_3, SrCrO_3 and SrMnO_3) can display very different behaviors. It also explains the occurrence of bad-metallicity in Sr- and Cr- oxides, and the recently reported record T_Neel of SrTcO_3. Last but not least these three aspects give a simple rationale behind the bad metallic properties of the normal state of Fe-based superconductors. L. de' Medici, PRB 83, 205112 (2011) L. de' Medici, J. Mravlje and A. Georges, arXiv: 1106.0815

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