Is feather edge bad?
As noted here 


  A thin edge is called a feather edge and can lead to preignition; the
  valve edge would burn away in a short period of time, and the cylinder
  would have to be overhauled again.


Basically during a rebuild over-grinding the valves can lead to a feathered edge which in turn can lead to higher/faster engine wear. Essentially by grinding the edge to thin you cause the limited remaining metal to wear out much faster. This can also lead to valve seating issues and out right leaks.