What physical traits are derived from the Neanderthal DNA in human DNA?
There was a very readable piece on this topic by Chris Stringer in Nature in 2012. At that time his general conclusion was:


  It is not yet clear whether the archaic DNA many of us carry is tied to any visible traits.


Although he added:


  More controversially, some of the known differences in coding DNA
  between Neanderthals and recent humans are associated with brain
  development and function


quoting the paper in Science (2010) in which the Neanderthal sequence was published. There may be more recent information of which I am not aware.

As to your question about Neanderthal DNA in different populations, it is not present in people of African origin, but is present in those of Europeans and Asian origin.