Some of the peculiarities of the atom scattering on metal surfaces are discussed in connection with the applicability of the Debye-Waller factor (DWF). The influence of the attractive potential well and of the finite size of the incident particles upon the scattering is commented. By comparing calculated DWF-values with experimental He scattering data it is concluded that the existence of diffraction is a necessary but not a sufficient condition for the applicability of the DWF. It is finally shown that the smooth potential surfaces which characterize the interaction between atoms and metal surfaces explain the apparently reduced thermal roughening observed experimentally.