What determines when an M4 is "worn out"?

I understand less wear is always better, but does the relationship between the BCG and the upper matter, or only to the extent that excess friction to the point of failure is not created during cycling, and that the bolt is guided into the locking lucks of the barrel extension?

What exactly are the "critical" wear areas (if any, besides the bolt/barrel extension/throat) in the M4/AR-15?