1) selecting from a list of reasons to down vote,
2) forming a jury of opinion or of a highly qualified judge,
3) rendering that verdict, and
4) having the poster or other protagonist option for challenging the down vote verdict.

1) Up votes add more points than a down vote subtracts to their recipients and if they were
   actually of equal but opposite value the reputation change for each would be equal but
   opposite.
2) A down vote on an answer costs the down voter reputation to assign but an up vote
   on an answer does not add to the assigner's reputation.
3) The reputation level for the privilege of first up voting (*circa* 15) does not equal
   that for first down voting (*circa* 125).